dozen (n.) Look up dozen at
c. 1300, from Old French dozaine "a dozen," from doze (12c.) "twelve," from Latin duodecim "twelve," from duo "two" (from PIE root *dwo- "two") + decem "ten" (from PIE root *dekm- "ten").

The Old French fem. suffix -aine is characteristically added to cardinals to form collectives in a precise sense ("exactly 12," not "about 12"). The dozens "invective contest" (1928) originated in slave culture, the custom probably African, the word probably from bulldoze (q.v.) in its original sense of "a whipping, a thrashing."