heat: [OE] From an etymological point of view, heat is simply ‘hotness’ – that is, the adjective hot with an abstract noun suffix added to it. But the addition took place a long time ago, in the prehistoric ancestor of Old English. The suffix *-īn ‘state, condition’ was tacked on to the adjective *khaitaz ‘hot’ to produce *khaitīn, which eventually became modern English heat. The verb heat is equally ancient, and was independently formed from *khiataz (het, as in ‘het up’, comes from a dialectal form of its past participle). => het, hot
Old English hætu, hæto "heat, warmth, quality of being hot; fervor, ardor," from Proto-Germanic *haita- "heat" (cognates: Old Saxon hittia, Old Norse hiti, Old Frisian hete, German hitze "heat," Gothic heito "fever"), from PIE *kaid-, from root *kai- "heat." The same root is the source of Old English hat "hot" and hæða "hot weather."
Meaning "a single course in a race," especially a horse race, is from 1660s, perhaps from earlier figurative sense of "violent action; a single intense effort" (late 14c.), or the meaning "run given to a horse to prepare for a race" (1570s). The latter word over time was extended to "division of a race or contest when there are too many contestants to run at once," the winners of each heat then competing in a final race.
Meaning "sexual excitement in animals" is from 1768, especially of females, corresponding to rut in males. Meaning "trouble with the police" attested by 1920. Heat wave "period of excessive hot weather" first attested 1890; earlier in reference to solar cycles. Heat-stroke is from 1874. Heat-seeking (adj.) of missiles, etc., is from 1956. Red heat, white heat are in reference to the color of heated metals, especially iron.
Old English hætan "to make hot; to become hot," from Proto-Germanic *haita- (see heat (n.)). Related: Heated (with many variants in Middle English); heating. Compare Middle Dutch heeten, Dutch heten, German heizen "to heat."
