Why is the ninth month called September?
late 14c., from Old French fermenter (13c.) and directly from Latin fermentare "to leaven, ferment," from fermentum "substance causing fermentation, leaven," from root of fervere "to boil, seethe" (see brew). Figurative use from 1650s. Related: Fermented; fermenting.
early 15c., from Middle French ferment, from Latin fermentum (see ferment (v.)). Figurative sense of "anger, passion" is from 1670s.
ferment fer·ment (fûr'měnt')
An agent, as a yeast, a bacterium, a mold, or an enzyme, that causes fermentation.