'in-heat' comes from the English phrase 'in heat.' The noun 'heat' originates from Old English 'hǣtu' (also written 'hætu'), meaning 'warmth' or 'heat.'

'heat' developed from Old English 'hǣtu' to Middle English 'hete' and then to modern English 'heat.' The figurative use of 'heat' to mean sexual excitement (as in animals) arose later, and the phrase 'in heat' was formed to describe animals in estrus.

Originally, 'heat' primarily meant 'warmth' or 'high temperature.' Over time, it acquired figurative senses such as intensity or excitement, and eventually the specific biological sense of sexual receptivity ('in heat').