1. sound noting confusion or disbelief
2. Posing the question, "what is going on."
Etymology: Likely to have come from Scooby-Doo often reacting to the order to go search for ghosts with Shaggy. Picked up in the late 90s.
Hot Girl: He wouldn't let me give him a blow job.
Guy: Aru??! He what!?
Bill: Come here, Joe. We need your help.