It's when one uninvolved and uninvited asshole barges in on a private conversation, and dominates it to the exclusion of one of the original people who was conversing. This can be a form of cockblocking.
Jason: John is a complete asshole. Notice how he is always stealing the conversation?
Joe: Yeah, what a fucking cockblocker!
Joe: Yeah, what a fucking cockblocker!
by Jimmy the Dick September 22, 2010