The quality or state of being cool; composure, calmness, or the absence of heat (sometimes used humorously or ironically).
Coined in the 1960s as a humorous back-formation from warmth, following the pattern that one might create 'coolth' as the opposite of 'warmth.' It plays on the slang sense of cool meaning composed or fashionable.
Coolth is a brilliant example of English speakers inventing new words through logical patterns—if warmth is the noun form of warm, why not coolth from cool? It was created humorously but has stuck around because it fills a gap in expressing composed confidence.
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