When a guy moves in on a girl that another man has began courting. Usually at a party/club/bar while the other man goes for a drink or to the bathroom but can also apply to any situation where a man has expressed interest in a girl and another man moves in without regard/respect. It is a sign of grievious disrespect and may result in shunning from the male group or a serious beating/pounding/rolling or other retribution. (also Cock blocker and cock blocking)
"I was talking to this girl and my buddy sat down on the other side of her and kept butting into our conversation trying to flirt with her, I'm gonna beat that cock blocker down!"