barm n.

also *bhreuə-, *bhreəu-, Proto-Indo-European root meaning "to boil, bubble, effervesce, burn," with derivatives referring to cooking and brewing.

It forms all or part of: barm; barmy; bourn (n.1) "small stream;" braise; bratwurst; brawn; brawny; braze (v.1) "to expose to the action of fire;" brazier; Brazil; bread; breed; brew; broth; broil (v.2) "to quarrel, brawl;" brood; effervesce; effervescence; effervescent; embroil; ferment; fervent; fervid; fervor; imbroglio.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhurnih "violent, passionate;" Greek phrear "well, spring, cistern;" Latin fervere "to boil, foam," Thracian Greek brytos "fermented liquor made from barley;" Russian bruja "current;" Old Irish bruth "heat;" Old English breowan "to brew," beorma "yeast;" Old High German brato "roast meat."

1530s, "frothing, covered with barm;" see barm + -y (2). The figurative sense of "excited, flighty, bubbling with excitement" is from c. 1600. The meaning "foolish" (1892) probably is an alteration of balmy (q.v.).