Derived from the trend of planking, bating, and owling. Originated in Chicago, IL; it involves the opening of ones mouth, a surprised facial expression, and the deliverance of a guttural utterance while standing perfectly still. The sound that is made resembles that of a vocally challenged person moaning aggressively with a closed throat.
Bob: "Hey Jim, what are you doing?"
Jim: "Well, I'm just about to go to the store to get some milk and *wailing* AHHHHUUUUGHHHHAHHHHH!"