'oestrous' originates from New Latin, specifically the word 'oestrus', which in turn comes from Greek 'oîstros' where 'oîstros' meant 'frenzy' or 'gadfly' (sexual excitement).

'oîstros' (Greek) was Latinized in New Latin as 'oestrus'; the adjective form 'oestrous' entered English in the 19th century to describe the state relating to 'oestrus'.

Initially, the root referred to 'frenzy' or a 'gadfly' (figuratively sexual excitation), but over time it developed the specialized biological meaning 'relating to the period of sexual receptivity in female mammals (heat)'.