It simply means that you don't believe what someone else said.
Mikey: Get the fuck outta here! What are you, nuts?
The interjection is also known to be a good palliative to heart attack when seeing unexpected unpleasant thing.
However it didn't work for OBL. He used it as a spell against the team six guys who got in his basement: " Get the fuck outta here!" but he get shoot anyway. The team six guys appeared to be deaf dumb both.