estrus (n.) Look up estrus at Dictionary.com
1850, "frenzied passion," from Latin oestrus "frenzy, gadfly," from Greek oistros "gadfly, breeze, sting, mad impulse," perhaps from a PIE *eis, forming words denoting passion (cognates: Avestan aešma- "anger," Lithuanian aistra "violent passion," Latin ira "anger"). First attested 1890 with specific meaning "rut in animals, heat." Earliest use (1690s) was for "a gadfly." Related: Estrous (1900).