The Collaborative International Dictionary of English v.0.48:

Withers \With"ers\, n. pl. [Properly, the parts which resist the
   pull or strain in drawing a load; fr. OE. wither resistance,
   AS. wi[eth]re, fr. wi[eth]er against; akin to G. widerrist
   withers. See With, prep.]
   The ridge between the shoulder bones of a horse, at the base
   of the neck. See Illust. of Horse.
   [1913 Webster]

         Let the galled jade wince; our withers are unwrung.
                                                  --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

withers
    n 1: the highest part of the back at the base of the neck of
         various animals especially draft animals