A ball buster is a guy who likes to bust your chops. Ball busters are very annoying. At first when they don't know you they are polite and pleasant. But after they get to know you for a day or two, they start taking the liberty to bust your balls making stupid sarcastic jokes about you all the time. At least 75% of American males are ball busters.
At work I avoid becoming friends with guys who I recognize as ball busters.