Acnestis

ac·nes·tis
[ak-nist-uhs]
-noun

The part of the body that cannot be reached (to scratch), usually the space between the shoulder blades.

Or possibly the back of the neck, when you don't have hands to scratch with..

Apologies for another swan blip, but I spent a lovely few minutes this afternoon on my way home watching these two preening.
This one really made me smile, as he (or she) tried so hard to reach those hard-to-reach places. Which I now know are Acnestis (acnestis' ? acnestises? acnestii?)