The dozens can be a harmless game, or, if tempers flare, a prelude to physical violence. But in its purest form, the dozens is part of an African-American custom of verbal sparring, of "woofin'" (see wolf ticket) and "signifyin'," intended to defuse conflict amicably, descended from an oral tradition rooted in traditional West African cultures. The dozens is a contest of personal power -- of wit, self-control, verbal ability, mental agility and mental toughness. Defeat can be humiliating; but a skilled contender, win or lose, may gain respect.
"Yo' mama," a common, widely recognized argumentative rejoinder in African-Amercan vernacular speech, is a cryptic reference to the dozens.
The term "the dozens" refers to the devaluing on the auctionblock of slaves who were past their prime, who were aged or who, after years of back-breaking toil, no longer were capable of hard labor. These enslaved human beings often were sold by the dozen.
(My wording from Wikipedia, the online encyclopedia.)
Two different origins for the name "dozens" are hypothesized: (1) that the term comes from the losing throw of 12 in craps, or (2) that it comes from the practice of selling retarded or damaged slaves "by the dozen" at slave auctions.