A dishonest, untrustworthy person; a scoundrel or rogue. Also the jack card in a deck of playing cards.
From Old English 'cnafa,' originally meaning 'boy' or 'servant,' related to German 'Knabe.' The meaning evolved from 'servant' to 'person of low character' to eventually mean 'dishonest person.'
The knave in playing cards is literally the 'jack'—in medieval times, a 'knave' was a servant, so the card showed a servant-like figure, which is why some decks still call it 'Jack' (a common servant's name)!
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