Barm is the foam, or scum, formed on the top of liquor – fermented alcoholic beverages such as beer or wine, or feedstock for hard liquor or industrial ethanol distillation – when fermenting. It was used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, has also been made from ground millet combined with must out of wine-tubs and is sometimes used in English baking as a synonym for a natural leaven.
barm n. Foam rising upon beer, or other malt liquors, when fermenting, and used as leaven in making bread and in brewing; yeast.
barm n. The lap or bosom.
barm n. foam that rises as yeast is fermenting (i.e. in bread leaven or malt liquor)