On most occasions, someone who, at a party, infiltrates a room where a couple is obviously trying to spend some time "alone". There's different variations of this type of character. Most of the time, an Undercover Blocker pretends to be asleep in the same room in order to be witness to teen-age sexual acts. Other times they often come in and out of rooms hoping to catch someone in the act. Any door that is unlocked is usually fair game for the Undercover Blocker.
Jon: Did Nick seriously pass out in that room?
Mike: I don't know man, I think he's just pretending in order to see that chick naked, or join in. No idea what his intentions are, man.
Jon: Wow, another Undercover Block. Nick is the freaking man.
Jeff: Dude, I just heard Jenny and Bob talk about going upstairs and making out in the bathroom. Wtf?!
Ian: Wow...Looks like I'm gonna have to go undercover tonight.
Jeff: What do you mean?
Ian: Undercover blocker... I'm going to go take a shit so they can't come in. Extra stank.
Jeff: Fucking genius.