Originally meaning when a person is about to engage in sexual activity and a person (family member, friend, child etc.) stops any chance of doing so. This can also be applied to any day to day non-sexual activity. Such as going out with friends, smoking bud, etc.
"Dude, I was guna get some ass, but then her best friend, who I think is a dyke and totally has a crush on her, cockblocked me."
"Fuck man, we were going to the movies, but then my mom cockblocked me."