Boo this man!
"Boo This Man!" is a clip from Half-Baked with Dave Chappelle. In the BBverse it is a part of Justin's soundboard (as one of the first pieces when the soundboard first appeared on BBLiveShow) and is played following an act of epic failure. Often the chat room will join in on the booing. (In NSFW #40, the audience really DID boo!)
Other variations of "Boo This Man!" include "Vuu This Man" (referring to vuvuzelas) which often results in the chat room flooding the buffer with long strings of "VUUUUUUUUUU."