We’ve all used this punch line. It is quite possibly the greatest punch line in the history of humor.
But is there a set up? I don’t think I’ve ever heard the actual joke that goes along with the line.
The version I heard is about a guy in a hospital describing someone else’s terrible injury, involving something highly uncomfortable going up the unfortunate victim’s asshole.
“Please,” said the nurse," the correct word is rectum."
“Rectum? It damn near killed 'im!”
There are zillions of variations but here is a sample joke that follows the typical pattern:
"Little Johnny’s teacher asked him how the weekend went.
He told her, “Horribly, a car hit my dog in the ass.”
She corrected him replying, “rectum.”
Johnny said, “Wrecked him? Damn near killed him!!”
It’s usually an ass joke, like a Little Johnny joke.
Little Johnny: My dad can hardly set down, he has hemorrhoids so bad on his ass.
Teacher: Rectum, Johnny, rectum!
Little Johnny: Wrecked him hell, it damn near killed him!
Not sure of the exact joke, the possibilities are pretty varied.
Not quite the same joke but…
Son of doctor: Dad, Dad, some local hoodlums just put a firecracker up Rover’s ass!!
Doctor: Rectum son, rectum.
Son: Wrecked 'em? Blew 'em clean off!!!
The setup I remember hearing first (although I’ve heard many variations over the years) involved Little Tommy relating the adventures of his pet turtle during Show and Tell. Tommy was riding his bike with his turtle in a basket on the handlebars. Tommy hit a bump, the turtle flew out, and Tommy’s bike tire got lodged in the turtle’s ass.
Teacher tells Tommy to use the word “Rectum”.
Punchline ensues.
The punchline is funny enough to apply to any number of ass jokes. I admire its versatility. 
{slight hijack: sometimes the “dirty little boy” of jokes is Johnny, sometimes Tommy. I’ve usually heard the tales of Tommy, you think the name are regional differences?}
In my world, it was always “Rectum hell! Damn near killed him”
Supercollider? But I just met her…
…
…
and then they invented the supercollider.
Similar joke from Boston:
Man is lying by side of road groaning. Cab driver sees him and puts him in the cab.
Man: Take me to the hospital"!
Cab: “Peter Bent”? (Peter Bent Brigham is the name of a hospital in Boston)
Man: “Bent?, she damn near bit them off”!
All those jokes are asinine, except for the last one, which had some teeth to it.
[d&r]
I love Humorbot 3000!
Richard Pryor does a version of the joke on one of his comedy albums. Little Johnny is telling his teacher about his brother in Vietnam, who got shot in the ass.
“Rectum, Johnny, rectum!”
“Wrecked 'im? It killed the motherfucker!*”
–Cliffy
*Motherfucker substituted for the word Pryor actually used.
Yep, definitely a Little Johnny joke.
A similar witticism is “liquor in the front, poker in the rear”
And, of course, “Poker? I don’t even know her!”
Little Johnny came to class one morning, crying his eyes out. The teacher asked him what’s wrong.
‘I was on my way to school,’ said Johnny, ‘And my dog was following me. He was out in the street and a guy came along in an old Model T Ford. My dog couldn’t get out of the way, and the Ford’s crank went right up his arsehole!’
‘Rectum!’ cried the teacher.
'Wrecked ‘im hell! It killed him!’
Sorry. This one didn’t rise to the level of a GQ.
Moved to MPSIMS.
samclem GQ moderator
Wait a minute. You substituted motherfucker as a euphamism?? 
For WHAT?
Anyway I like:
Slackjaw’d Yokel: I caught my wife in bed with my best friend.
Slackjaw’d Yokel 2: You bitter?
Slackjaw’d Yokel: Yup. Bit him too.
See, this works better than some of them. 'Cause some of them, it would be more likely to be corrected as “buttocks” or the like than rectum. To work it’s got to be the asshole . Just saying.
I say you’re right. It needs to be “asshole.”
I think it works best with Little Johnny (Little Tommy? – never heard of him before) speaking to his teacher. The teacher should say, “Rectum, Johnny, rectum.” (It sounds more like what a teacher would do if she says it twice. It also helps with the timing of the joke, giving more time for the word to sink in.)
Johnny’s reply should be written as “Wrecked 'im?” The whole point is that Johnny is NOT thinking or saying “rectum.”
Given that it was Richard Pryor, I’m guessing he dropped the nignog (the “n-word”).