Playing the dozens
A good natured, trash talking contest particularly between African Americans. It is part of an African tradition of "woofin" or "signifyin" and is generally seen as a nonviolent alternative to physical conflict. In a historical context "dozens" refers to old, brokendown slaves who were sold in the dozens.
The game, contest needs an audience in order to be successful, the audience can then cheer trach talkers on with cheers such as "snap" and "burn." Frequently these jokes take the form of "yo mama" but they can also refer to just about aspect of the offended and his or her life. However the goal is to not actually become offended; instead it is to be the most clever.


Below are a few links
Yo mama's so fat
Yo mama's so stupid
Yo mama has
Yo mama's so poor
Yo mama's so nasty
Yo mama's so ugly
Yo mama's so old