What’s the setup for this punchline?
I realize it’s fun to say whenever the word rectum is uttered, but where did it originate?
I’ve seen it on a list of “Redneck Medical Terms”. Along with such classics as:
urine: opposite of “yer out”.
dilate: to live a long time.
caesarian section: a neighborhood in ancient Rome.
Well, no doubt there are variations, but the way my old man told it (and note, the punchline is a little different) was something like this:
A bunch of guys were talking, and one said “You hear about Bill? He fell off the roof and landed right on his ass.”
The deacon was a member of the group, and he said "Don’t say he fell on his ass. Say ‘Rectum’. "
The first guy says " 'Wrecked ‘im’ be damned! It killed him!"
I think this joke works better on the more schwa-like sound of “wrecked 'em” rather than “wrecked 'im”, and that is how I heard it: as one of the many Little Johnny jokes…
First Day of School
Teacher: Now then class, tell us what you did on your holidays. Wendy, you can start.
Wendy: We went to visit Grandma.
Teacher: How sweet. Thank you Wendy. How about you, David?
David: Miss, my daddy took us to the circus.
Teacher: Very good. [turns to the evil Little Johnny who inhabits the back row of desks, and has been stretching his hand into the air Welcome Back Kotter style for so long that she can no longer ignore him] Very Well. Johnny… what did you do in the holidays?
Little Johnny: Aaaah Miss, we stuck bungers* up frogs’ arses and lit them!
Teacher: Rectum, Johnny. Rectum!
Little Johnny: It sure did, Miss. Blew their arses to fuckin’ bits!
[sup]*firecrackers designed for explosive, rather than visual effect.[/sup]
The thing about that, is it’s so clearly a punchline, not extremely risque, and still slightly funny without any leadup. Probably why it’s so often used on it’s own - typically to pretend someone’s been telling raunchy jokes when someone else comes into the room. 
Nanoda, I think you’re right. It’s up there with:
“I don’t think I could stand sixty-seven more of those!”
Help me out on this one… 
I first heard it at 68.The kid didn’t want any part of it after the first one.
Also “the shit’s in the front”,has been told as the shitty part/tracks are in the front.
Must be regional things.Or how gross you want to be.All I had to do was utter the punchline to an old friend of mine many years ago,and he’d crack up every time for months after I told it to him.
Anybody else think the OP’s sig was very… fitting?
This is one that’s better told than written,here’s the setup
Young virgin kid goes to cathouse for 1st experience.Hooker asks “whaddya want”. Kid I don’t know whatever you get for $50 (or whatever is today’s current price-at the time that was a goodsized fee)
Hooker-OK,let’s start with a 69.
Kid-What’s that?
Hooker-“just lie down on the bed I’ll teach you”
She lowers herself down onto him and cuts the cheese
Kid-and this is where the delivery comes in and also embellish the setup a little more.
** OK,OK,I can’t take 68 more of those**,or 67 if you want him to suffer a little more
“Twenty bucks, same as in town!”
That’s the one they always used to use on sitcoms and such. Anyone know if there’s a setup for it?
Priest: Sister, all over town today I’ve been hearing women on the street call out to me, shouting “Piece of ass! Twenty bucks!” Sister, what’s a piece of ass?
Nun: Twenty bucks, same as in town.
You may substitute a more arcane sexual innuendo, of course, but there are few bad punchlines that start with “And then the nun said…” 
Okay, this is the version I got from my dad.
Johnny and his little buddy were on their way to school. Being late, they decided to hop a picket fence and take a shortcut. Johnny made it ok, but little Jimmy got a picket right up his butt. Johnny hurried off to school, leaving him hanging, so to speak.
When Johnny got to school, the teacher asked him were Jimmy was.
“He’s back at the fence with a picket up his ass!” Johnny replies.
“Johnny! Don’t say ass! Say rectum.” the teacher says sternly.
“Rectum! Damn near killed 'em!”
Richard Pryor did a version on one of his comedy albums. The set-up was a student telling his teacher that his brother in Vietnam had just been shot in the ass.
–Cliffy