1850, "frenzied passion," from Latin oestrus "frenzy, gadfly," from Greek oistros "gadfly; breeze; sting; anything which makes one mad, mad impulse," perhaps from a PIE *eis- (1), forming words denoting passion (see ire). First attested 1890 with specific meaning "rut in animals, sexual heat." Earliest use in English (1690s) was for "a gadfly." Related: Estrous (1900).
word-forming element technically meaning "something produced," but mainly, in modern use, "thing that produces or causes," from French -gène (18c.), from Greek -genes "born of, produced by," which is from the same source as genos "birth," genea "race, family," from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups. First used in late 18th century French chemistry (see oxygen), it probably involves a misunderstanding of -genes, as though it meant "that which produces."