Made popular by a scene in the movie Black Sheep where Chris Farely's character is loudly heard telling the punchline. Abscence of the body of the joke in the movie led many to believe the joke did not really exist and there was only a punchline.
"Horrible, a car hit my dog in the ass," he said.
The teacher corrected, "Johnnie, we say 'rectum.'"
Little Johnnie replied "Rectum? Damn near killed him!"