Emily Dickinson Lexicon

BRASS, n. [Sax. bræs; W. prês; Corn. brest; Ir. pras. In Welsh, prês signifies brass, and what is quick, ready, sharp, smart, also haste, fuel, and presu, to render imminent, to hasten, to render present. The latter sense indicates that it is from the Latin. But I see no connection between these senses and brass. This word may be named from its bright color, and be allied to Port. braza, Sp. brasas, live coals, abrazar, abrasar, to burn or inflame; but the real origin and primary sense are not evident.]

1. An alloy of copper and zink, of a yellow color; usually containing about one third of its weight of zink, but the proportions are variable. The best brass is made by cementation of calamine or the oxyd of zink with granulated copper. – Thomson. Encyc.
2. Impudence; a brazen face.