c. 1500, "beggar's child" ("... wyle beggar with thy brattis ...), 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 the notion of "child's apron." Hollywood Brat Pack (modeled on 1950s Rat Pack) is from 1985. Brattery "nursery" is attested from 1788.
-y 2
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.