Reportedly, McAuliffe was raised in a household where swearing was harshly disciplined, so “Nuts”was his personal substitute for the usual expletives.. It was his G-3, Harry Kinnard, who suggested that become the official reply to the Germans.
FWIW, the Belgian Army still maintains the building where the 101st command post was located, and recreated the scene as best they could. It’s an excellent tour if you’re ever in the area.