A pissant is described by Kurt Vonnegut in his novel, Cat's Cradle, as follows:
"A pissant is somebody who thinks he's so damn smart, he can never keep his mouth shut. No matter what anybody says, he's got to argue with it. You say why you like something, and, by God, he'll tell you why you're wrong to like it. A pissant does his best to make you feel like a boob all the time. No matter what you say, he knows better."
That fucking pissant won't shut the fuck up!