My first submission! Please be gentle. Oh, and if you haven't seen The Aristocrats you probably won't get this.
George: A wizard walks into a talent agency and tells the agent that he has this great act for him. A family act; him, his wife, their kids, and their pet Crup, Petunia.
Fred: I thought you said that this was a dirty joke-
George: Hang on a tic, I'm getting there. So, yes, their act. "Well," the bloke says, "first my wife and I present the children to the audience in their finest little dress robes and they do little curtsies and bows and then they recite a heart-warming dissertationon on how Muggles are genetically, intellectually, and evolutionarily inferior and thusly should be purged from the face of the Earth by rabid Chimaera, in graphic detail. Then we bring a lush overstuffed armchair into the middle of the room in which I sit, with the little ones gathered round, as my lovely wife fetches me an issue of the Daily Prophet and a large decaunter of Fire Whiskey which I drink by myself in just under ten minutes while reading the latest scandal aloud to the little ones. After I finish, I grab my wife by the hair, force her to her knees, and then ride her like a virgin Thestral around the stage, whacking her backside liberally with the paper and calling her all manner of offensive sexual monikers, the kids sometimes join in too. "
Fred: Um, George-
George: Wait, I'm not done yet. Then the guy says, "After a few laps I dismount and have my way with her in all manner of scandalous positions, including, but not limited to, all manners of penetration with my monogrammed Nibus 2000, whilst the children huddle in the corner and bawl. I leave her as a floppy little rag doll centerstage and while still divested, beat my progeny soundly with a large dragonskin belt from my discarded trousers. Once everyone is mottled and bloody I berate them as I redress in a lavish blackrobe and then we all don Deatheater masks, light torches, and fetch a Muggle mother and child from the wings. We light the filthy woman on fire and roast her baby over it while sing the Dark Lord's praises.
Fred: George, you-
George: Fred could you just- "Then, to top it all off once it's skin is crackling and dripping we feed it to Petunia, whom then defecates and rolls around in it panting wildly. My son and I urinate on the fire to put it out and then the girls clean up the mess and are thankful for the opportunity. Then we all gather at the front of the stage and bow." The talent agent sits there a minute and then finally says, "That's one heck of an act-"
Prof. Snape: It certainly was. Perhaps you can ask Mr. Filch to borrow a detergent strong enough to clean that mouth of yours while you help with his duties for the next week. And ten points from Gryffindor, do you all use such filthy language outside of class? In the future you should at least save your dirty tales until you're in the your dormitories where you ought to be headed right now.
George: Bugger, Fred, you could have told me he was behind me.
Fred: I tried but you just kept on at it.
Fred: Well, it was rather, uh, interesting, I guess, but I just don't get it, mate.
George: Oh, right, so the agent asks him what the name of the act is, and he says, "The Aristocrats."
Fred: The what?
George: People of wealth and breeding, like the wizards that refer to themselves as "Purebloods."
Fred: They should call it The Purebloods then.
George: It's a Muggle joke. Yanks, I think.
Fred: That figures.