Nothing. Used to describe what a person is incapable of doing or what a person has no clue about either by learning or experience.

Broadly equivalent to Jack Shit, Diddly Squat, etc.
Guy1: I know that Obama is a communist and a muslim. He is also not a citizen.
Guy2: Yeah, what's your name?
Guy1: Jack
Guy2: Yea, Jack. Don't know Jack.
by blueliner49 February 07, 2010
A very common Australian nickname for police men.
Oh no, Jack! A jack just jacked my jacket. What a jackass.
by schase10353 December 30, 2009
Short for jackass
That dude is such a Jack, look at him.. Pathetic.
by Fishtown March 11, 2010
A type of kid that destroys at COD, usually lives life "legit", and knows that allmost everyone is full of BS. Usually somone who is afraid of small things, like kids he doesnt know, and doesnt like to do things that are too crazy. Also somone who keeps all there thoughts in there head so as not to attract a butt load of haters.
"Hey Jack theres some kids in your backyard"
"Yea ik bro but i dont want to go back there"
"why?, are you scared?"
"Nah, im just Jack"

or

"Hey Jack, do you like that one kid?"
"no hes so gay, he changes his story everyday, hes Mcleading it up bro"
"oh"
by whoareyouman May 17, 2010
D.C. slang for cigarette.
Ay slim, you got a jack I can burn?
by Remy3000 March 11, 2010
1. To masturbate
2. Alternative word for Nothing
3. On Your Own (An abbreviated form of "Jack Jones")
1. "Come on, who doesn't feel the urge to Jack one off when you see her on the telly?"
2. "I've done Jack all day"
3. "I'm on my Jack at the moment"
by Wo Wogan March 26, 2004
a name short for jack ass, but instead of using the whole word, you just shorten it to jack.
"let me pull over and let by buddy jack go"
by .fdl May 01, 2010

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