barm (n.) Look up barm at Dictionary.com
Old English beorma "yeast, leaven," also "head of a beer," from Proto-Germanic *bermon- (cognates: Dutch berm, Middle Low German barm), from PIE root *bher- (4) "to cook, bake" (cognates: Latin fermentum "substance causing fermentation," Sanskrit bhurati "moves convulsively, quivers," Middle Irish berbaim "I boil, seethe;" see brew (v.)).