A knave is a dishonest or untrustworthy person who deceives or tricks others for personal gain. In older English, calling someone a knave meant they were a scoundrel or rogue, someone who couldn't be trusted to keep their word or play fair.

You'll often encounter knaves in classic fairy tales and adventure stories, where they might be con artists trying to cheat the hero, or sneaky characters plotting schemes behind people's backs. The Knave of Hearts in Alice's Adventures in Wonderland gets accused of stealing tarts, which fits perfectly since knaves are known for taking what isn't theirs.

In playing cards, knave is another name for the jack, though today most people just call it a jack.

The word has an old-fashioned ring to it now. You're more likely to read about knaves in classic literature than hear someone called a knave in everyday conversation, though the concept certainly still exists. A classmate who copies your homework and claims it as their own, or someone who cheats their way through a game while pretending to play honestly, would have been called a knave in earlier times.