An accomplice; a person who helps someone else commit a crime or do something wrong.
From French complice, derived from Latin complex (past participle of complecti, 'to embrace' or 'to involve'). The meaning contracted to specifically mean someone involved in wrongdoing, likely through Old French intermediaries.
This is an archaic English form that lost a competition it didn't know it was in—French kept 'complice' but English evolved 'accomplice' instead, which sounds more decisive and Anglo-Saxon, better fitting English's preference for Germanic-sounding words.
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