Pygal

   Pygal is a zoological term meaning of or pertaining to the hind quarters of an animal.  The related word pygalgia is a rarely used term meaning pain in the buttocks.  I'm sure there is a joke in there somewhere since the expression pain in the buttocks, pain in the butt and pain in the a**e are versions of the expression pain in the neck meaning a nuisance; a source of trouble or annoyance.