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).