acnestis (ak-NES-tis) - n., in quadrupeds, that part of the spine (generally from between the shoulder blades to the pelvis) that the animal cannot reach to scratch.
The part that ALWAYS ITCHES and you CAN'T GET YOURSELF. If you're a quadruped, that is -- bipeds like most of us here are more flexible, generally speaking. The word comes, as you might expect, from ancient Greek: aknēstis, spine, from knestis, a cheese-grater -- and how it went from the whole spine to just a part in such a specific way, I'd love to know. Can anyone dish this out?
On the other bank, I spotted a tree with a branch that descended low enough I could scratch my acnestis, and lumbered across.