The part of the back (or backbone) between the shoulder blades and the loins which an animal cannot reach to scratch; the part of the human back between the shoulder blades.
rare in genuine use.
Mid 18th century; earliest use found in Robert James (d. 1776), physician and inventor of James's fever powder. From post-classical Latin acnestis from Hellenistic Greek ἄκνηστις spine, backbone from ancient Greek κνῆστις spine, cheese-grater, in Hellenistic Greek also itching (from -κναίειν to scrape, grate, scratch (only attested in compounds; ultimately from the same Indo-European base as Old High German nuoen to polish, Lithuanian knoti to peel off, flake away) + inserted -σ- + -τις, suffix forming feminine nouns), either by the addition of prothetic ἀ- or by false segmentation of κατὰ κνῆστιν ‘on the spine’ (Homer Odyssey 10. 161).