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."
The act or reactive measure when the opposite sex makes eye-contact with you and then proceeds to plug in their iPod as a defense mechanism to prevent you from making a move on them.
Josh: I saw this girl on the bus and was going to hit on her, then she busted out her iPod.