The Collaborative International Dictionary of English v.0.48:

Rancor \Ran"cor\ (r[a^][ng]"k[~e]r), n. [Written also
   rancour.] [OE. rancour, OF. rancor, rancur, F. rancune, fr.
   L. rancor rancidity, rankness; tropically, an old grudge,
   rancor, fr. rancere to be rank or rancid.]
   The deepest malignity or spite; deep-seated enmity or malice;
   inveterate hatred. "To stint rancour and dissencioun."
   --Chaucer.
   [1913 Webster]

         It would not be easy to conceive the passion, rancor,
         and malice of their tongues and hearts.  --Burke.
   [1913 Webster]

   Syn: Enmity; hatred; ill will; malice; spite; grudge;
        animosity; malignity.

   Usage: Rancor, Enmity. Enmity and rancor both describe
          hostile feelings; but enmity may be generous and open,
          while rancor implies personal malice of the worst and
          most enduring nature, and is the strongest word in our
          language to express hostile feelings.
          [1913 Webster]

                Rancor will out; proud prelate, in thy face
                I see thy fury.                   --Shak.
          [1913 Webster]

                Rancor is that degree of malice which preys upon
                the possessor.                    --Cogan.
          [1913 Webster]

WordNet (r) 3.0 (2006):

rancour
    n 1: a feeling of deep and bitter anger and ill-will [syn:
         resentment, bitterness, gall, rancor, rancour]