noun

1. a large beer mug, formerly made of leather coated with tar
2. black flag
3. ☆ a small, leather-covered bludgeon with a flexible handle
4. ☆ a SE U.S. scrub oak (Quercus marilandica) with fan-shaped leaves
5. ☆
   1. a gambling game at cards in which any player wins who gets cards totaling twenty-one points or less while the dealer gets either a smaller total or a total exceeding twenty-one points
   2. a combination of an ace and a face card or ten, equaling 21 points in this game
6. sphalerite, esp. a dark variety
   also written black jack

Etymology: see jack

transitive verb

1. to hit with a blackjack
2. to force (a person) to do something by threatening, as if with a blackjack