1400-50; (noun) late Middle Englishbrass(c)he a slap, crash, perhaps blend of brok(e) (Old Englishbroc breach, fragment, sickness; akin to break) and dasch smashing blow; see dash1; (adj.) in sense “brittle,” derivative of noun; in sense “hasty” by confusion with rash1
1824, of obscure origin, originally Amer.Eng.; perhaps akin to 16c. Scottish brash "attack, assault," or Fr. breche "fragments," especially of ice, from a Gmc. source (cf. O.H.G. brehha "breach," from brehhan "to break"), or to Ger. brechen "to vomit."