brat (n.)
c. 1500, "beggar's child" ("... wyle beggar with thy brattis ...), originally slang, from a 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 transferred meaning is perhaps from notion of "child's apron." Hollywood Brat Pack (modeled on 1950s Rat Pack) is from 1985. Brattery "nursery" is attested from 1788.