'estrous' originates from Greek, specifically the word 'oistros', where 'oistros' meant 'gadfly' or 'frenzy'.

'estrous' changed from the Greek word 'oistros' into Latin/Medieval New Latin 'oestrus' and then into English as 'estrous' (also spelled 'oestrous' in British English).

Initially, it meant 'gadfly' or 'frenzy' in Greek, but over time it came to denote sexual 'frenzy' or the period of sexual receptivity ('heat') in animals, which is its current primary sense.