when you first meet him, you think he's a cool guy. his personality takes over his very sub par looks. you go out with him and not only do you realize that the only thing he wants is his hands in your pants but that personality you thought was oh so great does not do anything to hide the face that he has been.. blessed with. he will cling on to you like gum on a shoe, and should come with a warning symbol. jack commonly refers to the fact that he probably jacks off to everything you say. oh, and he's a chick.
Girl: "Hey, what did you do last night?"
Friend: "Oh, I went to this really nice party. And, there was this guy there. But, he was a total Jack."
Girl: "Oh no, you poor thing. Where is he now?"
Friend: "I'm not sure. Probably jacking off to my facebook profile."