Everyone suffers an itch now and then. But when it is on that part of the back where you just cannot reach, no matter how you contort your body, it can get really annoying. You must have seen cattle rubbing their backs against tree-trunks and dogs writhing on the ground on their backs trying to relieve a persistent itch.
And would you believe it! There is a word for it! Seriously. It's called acnestis: the part of an animal's back where it cannot scratch itself.
Although this term is used with respect to quadrupeds (an animal that walks on four legs) nothing stops us from using it to refer to the same part of our anatomy.