late 14c. (intransitive), from Old French fermenter (13c.) and directly from Latin fermentare "to leaven, cause to rise or ferment," from fermentum "substance causing fermentation, leaven, drink made of fermented barley," perhaps contracted from *fervimentum, from root of fervere "to boil, seethe" (from PIE root *bhreu- "to boil, bubble, effervesce, burn"). Transitive use from 1670s. Figurative use from 1650s. Related: Fermented; fermenting.