1590s, "oppressively hot, close and moist" (of weather), ultimately from swelter + alteration of -y (2), either as a contraction of sweltry or from obsolete verb sulter "to swelter" (1580s), alteration of swelter. Figurative sense of "hot with lust" is attested from 1704; of women, "lascivious, sensual, arousing desire" it is recorded from 1940. Related: Sultriness.
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