acnestis

Acneſtis, from α priv. and κναειν to ſcratch. That part of the ſpine of the back, which reaches from the metaphrenon, which is the part betwixt the ſhoulder blades, to the loins. This part ſeems to have been originally called ſo in quadrupeds only, becauſe they cannot reach it to ſcratch.

1794, John Quincy, Lexicon Physico-medicum: Or, a New Medicinal Dictionary. Explaining the Difficult Terms Used in the Several Branches of the Profession. And in such Parts of Natural Philosophy, as are Introductory thereto. With an Account of the Things Signified by such Terms. Collected from the most Eminent Authors, 11th edition, London: Printed for T[homas] Longman, in Pater-noster Row, →OCLC, pages 16–17

ACNÉSTIS (ακνηςις, from α neg. and κναω to ſcratch). That part of the ſpine between the ſhoulder-blades, and which extends to the loins. It is ſo called from the difficulty there is to reach and ſcratch it.

1802, W[illiam] Turton, A Medical Glossary: In which the Words in the Various Branches of Medicine are Deduced from their Original Languages; Properly Accented and Explained, 2nd edition, London: Printed by J. D. Dewick, Aldersgate Street, for Lackington, Allen, and Co. Temple of the Muses, Finsbury Square, →OCLC, page 11

Excoriations are typically linear and occur where the patient can reach with his hands. […] The middle of the back is typically spared, although with a back scratcher, this rule loses value. This pattern of sparing has been called the reverse butterfly sign; the Greek word for the area that cannot be reached is acnestis.

2000, Otto Braun-Falco with Gerd Plewig, Helmut H. Woolf, and Walter H. C. Burgdorf, Dermatology, 2nd completely rev. edition, Berlin: Springer-Verlag, →ISBN, page 991

In what has to be the longest post-election season in living memory, the last five months have felt like an acnestis upon our collective soul; like that little patch of skin on our backs that we just can't reach to scratch ourselves. It's irritating. It's annoying. It's left us reaching and spinning around in circles.

2008 August 5, New Straits Times, Kuala Lumpur, Malaysia

I nudged Winston [a cat] to one side of his chair and sat down beside him. I scratched the spot between his shoulders, the acnestis that animals can't scratch by themselves, and he looked up at me and smiled.

2010, Blaize Clement, Raining Cat Sitters and Dogs: A Dixie Hemingway Mystery, New York, N.Y.: Thomas Dunne Books, →ISBN, page 24