cock block
A cock block is when two males are moving in the same girl and the fellow with no chance prevents the second male from advancing on the girl. He cock blocked the second guy and both lose.
This scenerio involves a male "friend" of a girl who continually tries to break the friend boundry while another male moves in on the girl. When the "friend" refuses to let the new guy and the girl be alone, he is cock blocking him. In a way, he is throwing the game for both teams, since no one wins... a cock blocker!