A non-threatening threat. Basically, it's how one will show their anger to another, quickly look around for a sharp object, fail in finding one, and say the threat anyways, not being able to back it up. Usually used in a friendly manner, where the speaker (probably) won't stab the other guy, anyway.
Jack: Hey man, you know what a pelican looks like!?
John: Well, you've got the saggy chins, the beady eyes, and you are annoying as Hell, so... I'm gonna have to say, like you.
Jack: I will stab you...