I’d definitely go with athletic or romantic:
Masturbation accident.
Don’t give any other details.
Just shake your head and say “I really hate muppets.”
Got it in on the first post, and, also: interesting OP/username combo. 
A while back I had just stepped into the shower when I remembered that I had wanted to take something out of the freezer to thaw. Figuring I’d have forgotten again by the time I was done showering and had gotten dressed, and also figuring that since I had only just stepped in surely I wasn’t *that *wet yet, I hopped back out and dashed into the kitchen. As soon as my (yes, wet) feet hit the kitchen tiles they slid out from under me and I slip-slided on my bare ass across the kitchen floor until I slammed into the oven. Much bruised ego, even more bruised skin.
I wound up telling my co-workers* exactly what happened. Your brother can borrow my story if he thinks it’s more dignity-saving.
“Wait. You were naked when this happened?!?”
“Well, I just stepped into the shower!”
Got my shoulder broken in a bar fight. Came down stairs the next day, pretending I was fine, went out to the porch to feed the cat, and pretended to trip over her. To this day, no one has ever doubted the story.
I know this gets said a lot, but this really is the best post/username combo ever.
First have them read this thread here.
Then tell them that one of the embezelled monkeys of the guy who was losing didn’t like what was going on and when embezelled monkeys get angry…well he got off cheap with only a gash on the head and a black eye!
Power went out when he overloaded a circuit, he went to the breaker box in the dark to turn it back on. After he opened it, he tripped on a dog toy, and smacked the corner of the breaker box with his face.
Defective eBay home surgery starter kit.
Rough sex.
Speaking of which ----- a friend of ours tried water skiing for the first time and while avid she wasn’t really good at it. She got wrapped in the ropes and dragged more than anything else. By arms, legs, torso and darn near the neck at one point. But she kept trying time and time again. The following Wednesday she had a gynie visit and wasn’t really thinking. Her Doc starts the exam and sees all these rope burns and bruises and asks her straight-faced “Well Chris - any changes in your sex life I should know about?”
I don’t get it. 
Someone 'splain, please? Googling “slothrop” brought up … more confusion, really.
[QUOTE=rachelellogram]
Power went out when he overloaded a circuit, he went to the breaker box in the dark to turn it back on. After he opened it, he tripped on a dog toy, and smacked the corner of the breaker box with his face.
[/QUOTE]
Damn, girl. I’m gonna have to remember that one.
Pretty easy to cut/scrape your forehead working under a car, or working on plumbing.
Also, “I was working on the house, and when I moved the ladder the (insert tool here) fell on my head.”
I’ve actually gotten a large papercut right in the middle of my forehead. I was carrying a huge stack of loose paper with both hands, trying to keep it from tipping over. When it started to tilt toward me, I cleverly headbutted the stack – and instantly regretted the idea.
I had a co-worker once whose husband was at home one day working underneath his tractor’s disk harrow when the cinderblocks supporting it shifted and it fell on him. He was pinned in place by the weight and got cuts on his face.
He couldn’t call for help (couldn’t reach the cell phone and no humans were within hearing of the remote place he was in), and lay under the thing for hours until she returned home from work and eventually found him. She was (understandably) mad as a hornet at him for some time.
It’s pretty obscure.
Tyrone Slothrop is a character in Thomas Pynchon’s Gravity’s Rainbow. TSlothrop’s post struck me as very Pynchonesque
I got a shiner and a cut-up bridge of the nose when I bent down to pick up something beside the bed just as the pitbull was jumping up on the bed. There was a resounding ‘thonk’ as our heads met. Their heads are pure concrete.
I did this, too, and made the mistake of telling the truth about it… my “friends” called me Lumpy foreeeever. 
Hilarious! I hope you don’t mind, but I fully intend to use this the next time someone wants an explanation for the scars between my eyebrows and on my chin from my recent bike wreck.
I’ve found that stories about waking up to find a refrigerator box full of iguanas, 2 dead hookers and a drug-crazed one-eyed mafia midget in my living room sometimes does the trick. Otherwise I just say I was hauling ass on my bike, hit gravel on a turn, blew a tire and plowed asphault face-first.
“yes, Mom… I did have my helmet on”
I just tell people my wife hit me in the head with my mini souvenier baseball bat while I was sleeping. Works every time.
I once was Pan Galactic Gargle Blaster drunk after a work function, and I woke up the next day with the knee of my pants shredded and bloody. Didn’t need stitches, and honestly I could have just hidden it without anyone finding out. I told some co-workers though, and they got a big kick out of it, because drinking to excess at work parties is sorta part of the culture here.
Sorry that anecdote related a lot less to the OP than I thought it would. slinks away
Seriously, if you make up a crazy story, then they’ll either realize you’re joking, or think you’re actually trying to cover up for something, and either way, they’ll come to their own conclusions about what really happened, and they’ll probably be worse than the reality (and likely involve domestic violence). Just say “I was drunk”, and leave it at that. Almost everyone has been stupid drunk at some time or another, and done stupid things. And the few people who haven’t, have still known plenty of folks who have. They’ll understand.