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.
The act of cutting short your gym workout because of prior commitments afterwards, only to find out that those plans have fallen through.
Eric totally jockblocked me the other day. I left the gym early to go meet him at the bar, but I got text saying he had to cancel as soon as I got there.
When you can't date, or have sex, with someone because they're the friend of someone you dated, or had sex with. Most commonly experienced when one wants to date one of an ex'sfriends, but they can't, because they're the friend of their ex.
"-I wanna date Jenny, but she's my ex's best friend, so I can't.
-You my friend, have been cockblocked by association."