As an adjective, it means slightly cold, but also can mean fashionable, impressive, or socially admired. As a verb, it means to become or make something less hot or less emotional.
From Old English *cōl* 'not warm', from Proto-Germanic roots related to coldness. The newer sense of 'fashionable' developed in the 20th century, especially in jazz culture.
The journey from 'a bit cold' to 'socially impressive' is one of English’s funniest meaning shifts. Calling someone 'cool' suggests they stay emotionally cool—calm, in control, not overheated. Slang often borrows physical words like this to describe social feelings.
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