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!!!.