As it happens, there was a documentary made in 2005 about this most vulgar of jokes, called, but of course, The Aristocrats. It's a lilting, buoyant movie that celebrates, not the gag itself or its ability to shock, amuse or offend, but the fascinating breadth and thrilling bravery of the unrestrained comic mind. (Digression: In the old courtly days, jesters weren't there only to make the king laugh. It also fell upon them to tell the truth and verbalize the uncomfortable, sort of like the boy who pointed out that the emperor had, in fact, not a stitch on.)
What exactly is this joke? Film critic Stephanie Zacharek gives us the lowdown:
The joke at the heart of The Aristocrats is an in joke among comedians, one they've historically told among themselves but never (at least until recently) in public performance. As at least one of the some 100 stand-up comics featured in the film puts it, the joke is a kind of secret-society handshake, a way for comedians to get laughs from their colleagues, the toughest crowd they'll ever face... [It's] "the comedy equivalent of jazz": Like a jazz standard, it's a gag with a predefined structure, or melody, that holds a universe of possibilities for improvisation.
The melody goes something like this: A guy walks into a talent agent's office and launches into a detailed account of his act, which involves himself, his wife and his kids, as well as, possibly, any number of family pets and assorted clever props. (Aunts and uncles, grandpas and grandmas, often join in the fun too.) The description of the act is the body of the joke, a transgressive shaggy dog story that may include, but is by no means limited to, lewd sexual acts of all sorts (incest, bestiality, you name it), plus blood and other types of bodily goo, and, ideally, a great deal of slippery, slidey shit and piss. The object of the joke is to come up with the filthiest, most offensive, most imaginative details possible. The booking agent listens to the windup and asks, in disbelief, "What do you call that act?" thus setting up the punch line: "The Aristocrats!"
Some of America's finest comedians (Robin Williams, Whoopi Goldberg, Paul Reiser, Drew Carey, Eddie Izzard, Sarah Silverman, George Carlin, Trey Parker of South Park) appear in The Aristocrats to do their take on the joke. That includes Bob Saget, the former host of the family-friendly America's Funniest Home Videos, who whips up "a version that gets so horrifically grisly in its details that it made me queasy," writes Zacharek.
However, the funniest version for me, bar none, is Mario Cantone's--for one reason. He does it a la Liza Minnelli, and it's about her sitting on top of a piano and fitting the whole thing up her vagina(!). What he says next is absofuckinlutely brilliant, hilarious, hysterical, as he morphs into Liza Z. down to the breathless voice, the husky notes and the slightly cross-eyed look (and he's not even in drag!).
I've seen this clip countless times, and I still die laughing every time, especially over the body part that "swells up into a large Macy's Day Parade balloon," which she then stretches out and flings over her shoulders--"the way Mommy used to do." Genius!