Good God Meaning

(idiomatic, Britain, US, mildly blasphemous, dated, Canada) Expression of surprise, outrage, or horror.

Example: 1885: Robert Louis Stevenson, The Strange Case of Dr. Jekyll & Mr. Hyde
  "Good God, sir," exclaimed the officer, "is it possible?" And the next moment his eye lighted up with professional ambition. "This will make a deal of noise," he said.