In pen and paper D&D, we call it “meta-gaming” - using information you have about the rules, that your character would of course not know, to decide what course of action your character takes, regardless of whether or not it fits with your character’s personality.
In other games - being cheesy, min/maxing or being a snert.
I play a variety of computer/pen and paper/board games, and the generic term in the U.S. I have always heard is “cheesy”: in fact, on the occasion where I use it to refer to athletic sports or even non-gaming aspects of life, the term is understood without a question even by most non-gamers. Of course, I’ve heard and even use a lot of the more rpg-specific terms as well.
In soccer (alright, alright: football) referees are taught to make calls within the “spirit of the game.” Hence, if a defender commits a heinous foul on a breakaway but the offence manages to get away, the attacker is given the “play-on” gesture (two open palms) and allowed to continue the attack. Sometimes, after the attack is over, you’ll see a ref give the offending (fouling) playing a yellow or red card well after the action has concluded. I always thought this was a brilliant rule. It hopes to negate the sort of hack-a-Shaq strategies that sully professional sports for the fans, the people who actually pay money to watch this lunatics in the first place.
Ah yes, one of the most famous duels of the renaissance. It was fought by Guy Chabot, the Baron de Jarnac and François de Vivonne, seigneur de La Châtaigneraye.
There are a few accounts of the duel which apparently was started when an acquaintance of Châtaigneraye made a disparaging remark about Jarnac, something about having a sexual relationship with his mother-in-law. Jarnac didn’t find the comment funny, and called Châtaigneraye a liar when he then went on to say that Jarnac had TOLD him about the relaitonship.
So, the duel was set for a month’s time.
Châtaigneraye was a skilled swordsman, and it appears Jarnac was not in his league. To cut a long story short, Jarnac was taken under the tutorship of one italian master, Captain Caizo. While he was taught all that he could learn in that short span of time, Jarnac did everything he could to delay the duel. He asked that there be brought several types of weapons, horses and armor, both varied and unusual so as to force Châtaigneraye to jump through some hoops to acquire everything Jarnac demanded.
In the meantime Caizo recommended that Jarnac force both parties to use a brassard on one arm which is a type of dueling arm protection consisting of a rigid plate of steel which forces the arm to lie extended and prevents bending at the elbow. This was probably to prevent Châtaigneraye from grappling Jarnac and disarming/throwing him to the ground and killing him there.
At this time he also apparantly tought Jarnac the famed maneuver. We don’t know exactly what it was, but it probably involved waiting for a time in which the leading leg was left open for an attack to the calf. As a fencer, I know this is dangerous, as any attack to a low target is. It probably involved good timing and good footwork, no doubt master Caizo did his best in this department.
The duel was fought with arming swords and bucklers (the brassard was used on the side in which the buckler was used so as to keep the arm extended), mail and helmets, and daggers for backup. Châtaigneraye was so confident that he even had an elaborate celebration dinner party all set up for after the duel.
In the end, however, Jarnac struck Châtaigneraye twice in the back of the leg, felling him. Châtaigneraye Tried to fight, even from his prone position. Jarnac asked the king to stop the duel, and after a while he relented.
Châtaigneraye, refused to declare himself a liar, and then refused medical care, and ended up bleeding to death.
So, the blow you were talking about, came to be known as the Coup de Jarnac.
Originally it meant only a very surprising move, that’s all. It wasn’t until much later that people began to use it as some sort of unsportmanly blow, probably trying to apply victorian sport fencing principles, or earlier dueling ‘rules’ to 16th century combat which was a different beast altogether.
That didn’t answer the OP, so much as gave clearer meaning to his example of ‘underhandedness’ 
The rules of a duel differed from time to time and form region to region, and heck, even from conflict to conflict. In this case you are right however. Jarnac called Châtaigneraye a liar, and Châtaigneraye challenged.
Remember also that this is the renaissance, not later periods were fencing was more martial sport, or meant only as part of the ‘gentemanly duel’.
Medieval and Renaissance martial arts encompassed ALL weapons and the grapple (loosely: striking unarmed and wrestling).
A fencer of the time, specially one who had seen military service as had Châtaigneraye, would not JUST know the sword. In fact it was said that Châtaigneraye was an excellent grappler, probably why Caizo recommended the Brassards.
Then my second example is the problem – “bush league”, AFAIK, definitely covers legal, but ungentlemanly, play.
Also AFAIK: in American football, blocking the kicker – hard – is totally legal on a kickoff return. This would not be immediately after the kick, but afterwards when the kicker has run down the field to ostensibly tackle the returner. It’s similar to laying out the quarterback after the QB has thrown an interception and the QB has become a defender – totally legal, but classless.
The Broncos linemen example – it would be illegal to ram a helmet specifically and maliciously into an opponent’s knees. However, it is legal to block defenders low in general when players are inside the “tackle box” (between the area occupied by both offensive tackles – IOW “the trenches”).
Not sure if Large Marge is still posting, but she vehemently maintained that she could sink this shot 50% of the time, and that was because she wasn’t very good at pool. 50%, you see, isn’t a very good success rate. Anyway, I greatly prefer ball in hand for the sole reason that a scratch should always be a great boon for the opponent.
The answer to the OP for all videogames, and especially online videogames, is “cheese”. A cheeser is a player who employs cheese tactics.
Some examples:
In a football game, finding a “perfect” place to move your defensive lineman before the snap that allows him to go untouched past the offensive line. Similar maneuvers can block kicks with unreasonable success. Other examples of cheese would be to call pass defense (dime) and audible to run (4-3) as the “perfect” defense, because computers tended to consider every warm body equally capable of tackling. And the most annoying is off-position players, like putting the RB in at QB to scramble up a storm, or playing WR1 at TE2 to draw a favorable coverage matchup. (Think Randy Moss covered by slow lumbering linebacker. Touchdown!)
In FPS shooter games, there’s a bunch, and some have their own terms. I forget the specific name, (nesting? no, but it had an E in it) but one common one was to just chill out by the spawning areas and kill all the newly spawned players before they could power up.
In RTS games, I recall certain rushes, which involve specific build orders, were considered cheese. Interestingly, unlike the other examples, most gamers felt this was a legitimate part of the game, and if you couldn’t overcome it, it was more the fault of your lack of skill than it was a flaw in the game design.
And finally, in the real world, there was the Pepsi song give-away, where you could tilt the bottles to read the cap before purchase, thus improving your 1:3 odds to 1:1 to win a song. I love this as a replacement phrase to “cheese”, which to my ear sounds like it was derived from a 12 year old’s leet speak. When employing cheese tactics, you are “tilting the bottle”. (And I consider the Pepsi thing to not even really be cheese, but that was a whole 'nother thread.)
In real-world professional sports, there is no such thing as cheese, but there is bush league. Bush league is when you take advantage of a situation to try and intentionally hurt somebody, or goad them into committing an ejectable retaliation. But cheese implies exploting an advantage to win, not injure. Winning is what these professionals are paid to do, so all advantages are fair game. You could call the Yankees and Lakers cheese organization for trying to buy championships, but it is a legitimate tactic as long as the leagues are okay with it.
Actually, there are cheese moves in professional sports, but they take place off the field. My favorite (uncited) story is the NFL player who claimed to send expensive call-girls over to the opponents’ hotel rooms the night before a game to wear them out. hehheh. Must be awful to be an NFL player, eh?
If you’re playing by any rules that are actually written down, it’s not legal. After a scratch, the next player has ball in hand, meaning he can place it anywhere on the table he wants and shoot in any direction.
Primal Rage, I believe.
I’ve only heard it called “spawn camping”.
There are several words I’d use for this, none of which are particularly suited for GQ.
Also, for those of you who don’t play (and I’ve been out of the loop for some time, myself): The multi-hundred-dollar cards Critical1 is talking about aren’t often used in high-level play these days. These cards are from the early days of Magic, when the designers weren’t entirely sure what a fair cost-to-effect ratio on certain cards were. And most high-level tournaments only allow cards from the last three or so expansions. I think the only card printed recently that’s worth $100+ is a foil Birds of Paradise.
I’m originally from Texas. On return visits I sometimes play some 8-ball at bars. Apparently they have their own set of rules for the game (and a few strange ideas about 9-ball, as well). I was accused of playing “squirrelly”. Some perfectly legal shots of mine were called “squirrel shots”. One guy grabbed a ball before it could reach the pocket (which would have made it unretrievable on the coin-op table) and insisted on spotting it, and that I forfeit my turn. Another warned me that some people would get violent over such violations. The problem was that these rule changes were not consistant. Players will argue at the table about rules that have probably never been written down.
My solution to this is to tell opponents in advance that I will be playing by BCA (Billiards Congress of America) rules and that if that’s not OK with them then we don’t have to play.
To answer the OP, I call it “fair play”. The rules are there to make the game fair. If all players play by the same set of rules, how can it not be fair? If people don’t like a particular strategy, and think it takes something away from the game then a new rule is needed. Codification (rather than informal agreement) is advisable, however, to avoid the problem of “Texas rules”.
My pardon, I thought you were talking about running into/roughing the kicker in your first example. IIRC, it is not legal to block the kicker until he crosses the line of scrimmage, or the ball has been touched by either team.
I’d point out that the QB can usually count on getting blocked after an interception. It would a rare team that did not take the opportunity to wind him a bit. I agree with your characterization of ‘laying him out’ being unfair, though. The only recent example I can vaguely recall was flagged for unnecessary roughness (with Favre as the hittee? I don’t remember).
I called your second example illegal because you cannot lead with your helmet in any tackle. It’s most often cited when a DB hits a receiver, but it’s a 15-yard penalty for a “tackler [to] us[e] his helmet to butt, spear, or ram an opponent.”
Pushing the envelope. Brinksmanship.
That originated at the University of Nebraska and my beloved Sooners were once the victims of that play. The name of the play was a reference to the large native-grown linesmen Nebraska used and their often slavic ancestry.
Their is nothing llegal about it today and no one thinks such plays in American footbal are sneaky or underhanded. The “trick play” is a normal and accepted facet of the game, the most notable examples being the “student body” reverse, halfback pass, and eligible-reciever lineman plays, not to mention fake punts and field goals.
ARRRRRRGFGHFPTHFGPTGFG!!!
goes batshit crazy
Consider the example in the 2002 NFL Wildcard game, Giants at 49ers. Rich Seubert, an offensive guard, notifies an official he is an eligible receiver on a routine (albeit game winning) FG attempt late in the game. Bad snap. Holder picks up the ball and starts yelling “FIRE! FIRE!”, which tells his teammates “botched snap, go out for a pass!”
Seubert rumbles downfield, gets open, turns around looking for the pass. Here it comes, right to him, wide open. Oh no, a defender mugs him, and drags him to the ground before the ball gets there. Clear pass interference, which entails 1st and 10 from the spot of the foul and one more play. You see, the clock had expired, but a defensive penalty extends the game for one more play. Giants will surely win now, right? It’s a chip shot from this distance.
Wrong. The ref calls “illegal man downfield, so there was no pass interference penalty.” Game over. Sorry Giants, you go home, but that sure was a fine season you had going. Too bad, so sad.
Later, “oh we’re sorry, yes the Giants should have gotten another play. Our bad.”
ARRRRRRGFGHFPTHFGPTGFG!!!
The moral of this story is to be careful of trick plays, as you can sometimes be too tricky for your own good.
Actually, the moral would be “sometimes the zebras suck”. The alternative was for the Giants to attempt a difficult field goal. Some days you eat the bear and some days the bear eats you.
Rugby has a still better variant of this. If the referee sees that the non-offending side is doing OK despite the infringement, he makes a mental note of the site of the offence, calls (say) “Knock-on! Advantage!” or “Offside! Advantage!” and signals the appropriate punishment (scrum or penalty), but does not stop play. If the non-offending side then fails to accrue any advantage, or commits a minor offence of their own, the referee blows up and awards the penalty or whatever. But if they do actually gain a certain advantage proportionate to the severity of the offence, he calls “Advantage over!” and play continues.
F’rinstance, a knock-on (fumble) will ordinarily entail a scrum at the place of infringement, but if having gained possession the non-offending side gain ground or put away a good kick of their own, the ref will call “Advantage over!” as right has prevailed anyway. The point is that he can wait a few moments - even up to half a minute - to see which way the wind is blowing.
In a general sense in answer to the OP and not the more entertaining dueling or fumblerooskie discussion I think the word is either “unscrupulous” or “extra-legal.”
Historically, Rugby has seen its share of “cheesy” plays that have subsequently been outlawed:
About a hundred years ago, the All Blacks (New Zealand’s national Union side) worked out that seven men could hold their own effectively against eight in a scrummage. This entailed having only two men in the front row (which engages the front row of the opposition’s scrum-formation rather like interlacing the fingers of your two hands) and having them bind with their heads together, not allowing the opposition to interlace. That left only their two outside shoulders to bind against, which meant in effect that the third man in the opposing front row had no-one to push against. This achieved, the Blacks had an extra man available outside the scrummage, which is a big advantage. The Laws were soon changed such that a three-man front row was mandatory, and so was interlacing.
In the Seventies, the Wallabies (Australia) developed an unusual “short penalty” move. (A penalty kick can just be “tapped” an inch or so to confer free possession, the opposition being obliged to retire ten yards while you do this.) Their forwards waited a short distance from the kicker with their backs to the opposition and on receiving the ball, they transferred it from hand to hand temporarily out of sight. Then one of them, previously chosen, hid it up his jersey while the others stuffed their empty hands up theirs. In Rugby you can’t tackle someone who hasn’t got the ball… and trying to work out who had it while eight men ran in eight different directions was a real brain strain. Again, this is now illegal.
Penalty kicks at goal are a little like American field goal attempts, except that the ball is just teed up and the defending side must wait passively until the ball is kicked. High-quality kickers can land 'em from up to fifty-something yards away. As the crossbar is ten feet high, no-one has a hope of intercepting even a long-range attempt that is just creeping over the bar… unless you station a six-foot-six defender with two other tall strong men to launch him into the air as the ball arrives. Once again, spoilsport lawmakers have outlawed this tactic. 
back in the early days of mtg I used to twist the rules seriously hard, I was in direct contact with the head guy in charge of card rulings. my most famous one was the tounament leagal use of a garbage can lid to destroy my oppnents cards in play. (well leagal at the time) that deck changed the rules on several cards as well as creating some new ones. sportsmanlike? not really but it did improve the game in the long run.
I think it would also depend on if it became a standard tacic. Don’t remember if the ‘Fosbery (sp?) Flop’ was considered bad form at the time but it surley is not now.
In high school football, US, we had a desperation play in our book. At the line up the QB would step back point to the clock, everyone followed suit, when our opponants turned aroung to see what we were pointing at we snapped and continued the play. I never considered it ‘cricket’, but it did work…and was completely legal.