One who prevents a guy from banging a girl. Usually a friend or family member, motivated either by jealousy or the fact that they can't get any and don't want to see you get some.
I put the moves on Jane last night but her protective cockblocker brother aggressively cockblocked me and asked me to leave the party as he didn't want me to get any.
A woman/girl (usually ugly) who is jealous of the fact that her friend gets dick, whilst she doesnt. thus she tries to block all cock directed at friend. the ugly bitch.