From Middle English barm, barme, berm, bearm, from Old English bearm (“lap; bosom”), from Proto-West Germanic *barm, from Proto-Germanic *barmaz (“lap; bosom”), from Proto-Indo-European *bʰer- (“to bear”). Cognate with German Barm (“lap; bosom”).
From Middle English berme, berm, from Old English beorma, from Proto-West Germanic *bermō (“yeast; barm”); related to the dialectal Low German Bärm (“yeast”), from Middle Low German barm, berm. The cake sense is possibly a shortened form of barmcake, which would be made with yeast as described in that sense, or possibly it is from the Irish bairín breac, a type of bread.
Meanings
(countable, dialectal, obsolete, uncountable) Foam rising upon beer or other malt liquors when fermenting, used as leaven in brewing and making bread; yeast.
In 1577 yeast, called barm, is bought at 9d. the pail. 1882, James Edwin Thorold Rogers, A History of Agriculture and Prices in England, volume 4, page 620:
And he chaffed the women as he served them their ha'porths of barm. 1913, D[avid] H[erbert] Lawrence, Sons and Lovers, London: Duckworth & Co. […], →OCLC:
[A]nd sometimes make the drink to bear no barm. c. 1595–1596 (date written), William Shakespeare, “A Midsommer Nights Dreame”, in Mr. William Shakespeares Comedies, Histories, & Tragedies: Published According to the True Originall Copies (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act II, scene i], line 25:
(countable, dialectal, obsolete, uncountable) A small, round, flat individual loaf or roll of bread.
fr mousse (foam rising upon beer, etc., when fermenting) · ga giosta (foam rising upon beer, etc., when fermenting) · fi vaahto (foam rising upon beer, etc., when fermenting)