YGBSM... a colorful saying which in no uncertain terms questions what was just said. I have myself used this in many occasions intentionally, and a few instances where I was so overwhelmed with what I was told, and it caught me off guard, and I just blerted right out. "You've got to be shitting me". And on rare occasions with a very colorful but inappropriate verb inserted to emphasize my shock & awl. So now you know...
As to the origins or how any concept of the literal meaning of the phrase? Well, you got me. My grandfather said it (among other things), my father said it (also among many other things), and I said it throughout my life... and never gave any thought to the literal wording if the saying. Didn't have to... because whenever it was used, the other person knew exactly what you meant. Go figure.
