Emily Dickinson Lexicon

BARM, n. [Sax. beorm. Qu. L. fermentum, from ferveo; or beer-rahm, beer cream; or W. berwi, to boil.]

Yeast; the scum rising upon beer, or other malt liquors, when fermenting, and used as leaven in bread to make it swell, causing it to be softer, lighter, and more delicate. It may be used in liquors to make them ferment or work. – Johnson. Encyc.