Definition of jack inBritish, informal
- He was building a new shed but jacked it in and watched the ball game.
- He jacked in his job and became a hippie.
: a game played with a set of small objects that are tossed, caught, and moved in various figures
: a small 6-pointed metal object used in the game of jacks
: a playing card carrying the figure of a soldier or servant and ranking usually below the queen
: to move or lift by or as if by a jack
: to raise the level of
: to take to task
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