Word Origin & History
c.1500, slang, "beggar's child," originally northern, Midlands and western England dialect word for "makeshift or ragged garment;" probably the same word as O.E. bratt "cloak," which is from a Celtic source (cf. O.Ir. bratt "cloak, cloth"). The modern meaning is perhaps from notion of "child's apron."
Bratty is first recorded 1961. Hollywood Brat Pack (modeled on 1950s Rat Pack) is from 1985.