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1.
the buildup of seminal fluid from jacking off in the shower that prevents the shower from drainng.
beacuse of the fuckin cumblock the shower takes four fuckin hours to fuckin drain
by kyle m July 16, 2003
 
2.
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."
by mr.seed May 11, 2008
 
3.
The act of walking in on your mate or male having sex with their girlfriend/wife as they are about to cum or nearing the stage where as they would usually cum. Thereby preventing them cuming. This is also similar to the legendary Cockblock.
Sam: Dude I can't believe u Cumblocked me I was so freaking close arr.
Rory: Im sorry man, how could I know?
Sam: Well walking in for a start would have helped!!!.
by Sammyb01 May 11, 2008