Acnestis [æk'ni:stIs // ak'nee-stis]
That part of the back between the shoulder-blade and the loins which an animal cannot reach to scratch.
From the Greek άκνήστις, meaning spine.
1927 Observer 3 Apr. That spot known to crossword solvers as the acnestis.
There's a word for the dent in your upper lip (philtrum), the space between your eyes (glabella) and the corners of your eyes (Canthus), but the name of the bit you couldn't reach on your back has always eluded me, right on the tip of my tongue, right in the acnestis of my back!