Not fastened with buttons; also means relaxed, casual, or speaking openly without formality.
From English 'un-' (prefix meaning 'not') plus 'buttoned' (from Middle English 'boton,' derived from Old French 'botoner'). The literal meaning dates to medieval clothing, while the figurative 'relaxed' sense emerged in 20th-century American slang.
The shift from 'unbuttoned' as a fashion description to meaning 'relaxed and talkative' is a perfect example of how our clothes shape our language—buttoned-up people were formal and controlled, so unbuttoned became synonymous with freedom and informality.
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