'oestrus' originates from New Latin, specifically the word 'oestrus', where the root comes from Greek 'oistros' meaning 'gadfly; frenzy (sexual excitement)'.

'oestrus' came into English via New Latin from Greek 'oistros'; the word also appears as Middle English/early modern variant 'estrus', and eventually the modern English form 'oestrus' (also spelled 'estrus') was used.

Initially it referred to the Greek sense of 'gadfly' or 'frenzy', and over time the meaning narrowed to the biological sense 'the period of sexual receptivity in female mammals' used today.