'estrus' originates from New Latin, specifically the word 'oestrus', where Greek 'oistros' meant 'gadfly' or 'frenzy'.

'estrus' changed from the Greek word 'oistros' into New Latin 'oestrus' and then into modern English as 'oestrus' and the variant spelling 'estrus'.

Initially, it referred to 'frenzy' or 'gadfly' (figuratively a state of agitation), but over time it narrowed to mean the specific biological condition of sexual receptivity in female mammals.