Similar to a cock block, this is the act of walking in someone (usually a male friend) while they are having sex, to stop them from making it all the way.
Guy 1: "I was rooting this chick the other night, I was about to blow but my mate walked in and totally cum blocked me."
Guy 2: "Tough luck man."
Guy 2: "Tough luck man."
by mr.seed May 16, 2008
Get the cum blockmug.