c.1500, slang, "beggar's child," originally northern, Midlands and western England dialect word for "makeshift or ragged garment;" probably the same word as Old English bratt "cloak," which is from a Celtic source (compare Old Irish bratt "cloak, cloth"). The modern meaning is perhaps from notion of "child's apron." Hollywood Brat Pack (modeled on 1950s Rat Pack) is from 1985.