A verbal insult game popular among African Americans. The exact origins are unknown but it appears to draw heavily from African oral traditions. The style can be "clean" or "dirty". Clean dozens jokes usually attack appearance or intelligence or style of dress. For example:
"Your sister is so ugly, she'd make a freight train take a dirt road."
Jokes about one's mamma (and/or sexually inspired jokes)are usually considered "dirty dozens":
"Your momma so fat, she broke her leg and gravy came out"