This term is used to describe someone who thinks they know everything yet are often wrong. They will argue and lie to be right. Often get louder when challenged. It can also be said they have shit for brains. Often offer unnecessary information just to have something to say.
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.
When you is Jack and you pour hydrofluoric acid down your toilet to fix a clog and shit comes spraying back up in your face giving you acid burns that need medical attention.
Bob didn’t know jack shit could happen until he woke up the next day in the hospital smelling like burntcrap.