Having an evil, corrupt, or obscene mind; inclined toward sexual or morally inappropriate thoughts.
Compound of 'foul' + 'minded' (from Old English 'gemynd' meaning memory or intention). This formed as a fixed phrase around the 16th-17th centuries to describe someone's internal character.
Before modern English standardized compound adjectives, people would have written this as 'foul of mind' or 'foul minded'—the hyphenation convention that emerged actually helps us understand that it's describing a quality of someone's mind, not their literal face or body.
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