Sophisticated, refined, and well-mannered; the opposite of 'uncouth.' As a noun, it means sophistication or good manners.
A back-formation from 'uncouth' (Old English 'uncūð' meaning unknown or strange), created in the 20th century when people realized they could invent the opposite by removing the prefix 'un-.'
Couth is a perfect example of how English speakers are language engineers—it didn't exist until someone thought, 'Hey, if uncouth is a word, shouldn't couth be too?' and invented it, even though it sounds slightly made-up.
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