Source : Webster's Revised Unabridged Dictionary (1913)

Rancor \Ran"cor\, 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.

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

   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.

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

                Rancor is that degree of malice which preys upon
                the possessor.                    --Cogan.

Source : WordNet®

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