Someone, usually an intern or less gifted coworker, who doesn't stop talking to a colleague or superior about a topic you wish to speak about, but because they sound pathetic and annoying, you realize you can't possibly jump in the conversation or follow up without also sounding annoying, even when you know your are smarter, more relevant and usually better looking. And thus your talk has been blocked.
Say your new to a job and want to make an impression to your new boss, so you get ready to talk to them about something you really know, and then this older intern who was hired to organize folders on the servers and punch holes in paper, starts blabbing about the same thing to your boss, and you see they want to run away, and you're like oh shit, I can't follow that or the boss will hate me too. Damn you talk-blocker, damn you. Of course you realize you are getting paid and he is not, so that's cool.