noun: The part of the body where one cannot reach to scratch.

From Greek aknestis (spine), from Ancient Greek knestis (spine, cheese-grater).

“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.” –A Wish List to Soothe Our Collective Itch; New Straits Times (Malaysia); Aug 5, 2008.

Via A.Word.A.Day.