Acnestis

Definition: “The part of the back (or backbone) between the shoulder blades and the loins which an animal/person cannot reach to scratch” (Oxford English Dictionary) This lovely word is not often found; one of the few dictionaries that does define it, the Oxford English Dictionary, notes that it is “rare in genuine use.” You may use this word in any fashion you see fit. In fact, your need of it doesn't even need to be genuine.