A "Death Cube", all Vernor Vinge references aside, is the epithet placed upon a room full of people with nothing exciting to do, and no viable plans for the night. A death cube is often accompanied by awkward silence, sexual stagnation, and, as the colloquialism implies, death of all things enjoyable.
"Yo, the party's not happening."
"Are you serious? Shit."
"Well, do you have anything else in mind for tonight?"
"This is a death cube."
"Yo man, O.D. Death Cube. We need to get out of here and do something FAST."