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coolth (n.)[coolth 词源字典]: 1540s, from cool on the model of warmth. It persists, and was used by Pound, Tolkien, Kipling, etc., but it never has shaken its odor of facetiousness and become standard.[coolth etymology, coolth origin, 英语词源]
facetious (adj.): 1590s, from French facétieux (16c.), from facétie "a joke" (15c.), from Latin facetiae "jests, witticisms" (singular facetia), from facetus "witty, elegant, fine, courteous," which is of unknown origin, perhaps related to facis "torch."

Formerly often in a good sense, "witty, amusing," but later implying a desire to be amusing that is often intrusive or ill-timed. Related: Facetiously; facetiousness. "Facetiæ in booksellers' catalogues, is, like curious, a euphemism for erotica." [Fowler]