Cock block can happen when you are already talking to a girl(or guy) of interest alone and someone comes up and starts to talk and hang out with you.
2) Cock block can also happen litterly, when someone prevents you from having sexual intercourse with a female of interest.
2) Matt and Kaitlyn were about to have sexual intercourse when some drunk walked into the room and cock blocked Matt.
2) The act of "cutting" at a 4-way stop sign and "cock-blocking" someone who has waited thier turn at a stop sign.
Girl 1: I don't have to go home tonight.
Guy 2: Hey Carl, remember that time you shit yourself?
Girl 1: Ummm... I have somewhere to be...
2) This dumbass in a drivers ed car just cock blocked me at the stop sign. So I tailgated the fucker till he ran of the road, it was my turn at the stop sign, he needs to learn his place.
"Fuck man, we were going to the movies, but then my mom cockblocked me."