The Dozens is a game that has its origins in African American slavery. The game originates from the devaluing and bargaining off of deformed or defective slaves in auction houses. This element of the African American oral tradition in which two competitors, usually males, go head-to-head in a competition of often good-natured insults. They take turns "cracking," "snapping," "ribbing" or insulting on—one another, their adversary's mother or other family member until one of them has no comeback. This is called playing the dozens or doin' the dozens, and sometimes dirty dozens. The dozens is a contest of personal power—of wit, self-control, verbal ability, mental agility and mental toughness. Each putdown, each "snap", ups the ante. Defeat can be humiliating, but a skilled contender, win or lose, may gain respect. The dozens is one of the contributing elements in the development of hip hop, especially the practice of battling.
The dozens can be a harmless game, or, if tempers flare, a prelude to violence. While the competition, on its face, is usually light-hearted, smiles sometimes mask real tensions. But in its purest form, the dozens is part of an African-American custom of verbal sparring, of woofin' and signifyin', intended to defuse conflict nonviolently, descended from an oral tradition rooted in traditional West African cultures.
"Yo mama", or "yo madra" are common, widely recognized argumentative rejoinder in African-American vernacular speech, is a cryptic and sometimes comical allusion to the dozens. An example would be, "Yo momma is so fat she follows Burger King on Twitter!"
http://en.wikipedia.org/wiki/The_Dozens
