I'm not sure if "hot" as "warm" or "heated" existed before "hot" came to mean "good-looking" or "attractive", but if so, how did this new meaning come to be?
According to OED 1, hot in the primary sense is 'common teutonic'. In the general sense of 'having or showing intensity of feeling; fervent, ardent, passionate, enthusiastic, eager, keen, zealous' OED's earliest citation is dated to 971, and Chaucer uses hot of sexuality before 1385:
It's obvious that what gets you hot may be regarded as hot itself; and in this transferred sense we again find Chaucer writing of hot spices. It's at least arguable that it's in the sense of 'exciting lust' that the Prose Merlin (ca. 1450) has:
The exact meaning here is debatable; I suspect it means 'passionate' or 'lustful' rather than 'sexually attractive';
EDIT I'm going to withdraw that last as superfluous, since for a woman to be 'hot' in the sense of 'lustful' or 'willing to copulate' is in itself an attraction, and has been for centuries:
From etymonline for hot, “The association of hot with sexuality dates back to c.1500.” Also, “Sense of "exciting, remarkable, very good" is 1895 ... Hot stuff for anything good or excellent is by 1889.” The etymonline entry isn't clear about the dates of “warm” or “heated” meanings, but those apparently predate the other. OED 1 cites for the word in various senses of heated materials, or in forms like “hottest”, date from 1000 AD, 1200, 1250, 1300 etc; but the vowel frequently was a rather than o into the 1500's.
Also see the etymonline entry for red-hot, which notes “Red-hot mama is 1926, jazz slang, "earthy female singer," also "girlfriend, lover."”
The term "hot" has been so-long intertwined with sexuality that it practically predates writing. Despite the fine scholarship that has responded so far, I think we must look to the roots of the connection between "hot" and sexuality:
protected by tchrist Oct 1 '12 at 3:49
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