Habitually using offensive, vulgar, or abusive language; prone to swearing or saying obscene things.
Compound adjective of 'foul' + 'mouthed,' following the pattern of other '-mouthed' compounds like 'big-mouthed' and 'loose-mouthed.' This formation became standard by the 17th century.
The compound 'foul-mouthed' is interesting because 'mouth' isn't actually the problem—it's the words coming out of it—showing how English metaphorically associates the physical mouth with the speech it produces.
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