1) Traditionally refers to drawing Jack-Deuce as your starting hand, ie. "Jack-Shit"; a bad hand. Although may sometimes be extended to refer to a combination of a Jack with any low value card 2-7, typically.
2) Jack-Shit Poker - A variation on the familiar Seven-Deuce Hold'em. A pre-arranged additional reward is owed by one's opponents if he wins a hand with Jack-Deuce as his hole cards.
Player X: You had a straight?
Player Y: Nah, I had Jack Shit!