To damage someone's reputation or make something less pure or clean.
Possibly from Old French 'souiller' (to soil), from Latin 'suculare.' The sense evolved from physical dirtying to damaging someone's character or honor.
The phrase 'sully one's name' shows how closely we tied reputation to identity—in medieval times, a family's honor was more valuable than money, so sullying someone's reputation could ruin their entire future and social standing!
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