n (card games) A common gamblingcard game in casinos, where the object is to get as close to 21 without going over.
n (card games) A hand in the game of blackjack consisting of a face card and an ace.
n The flag (i.e., a jack) traditionally flown by pirateships; popularly thought to be a white skull and crossedbones on a black field (the Jolly Roger). In older literature sometimes spelled "black jack".
n a small, flat, blunt, usually leather-covered instrument loaded with heavy material such as lead or ball bearings.