The Collaborative International Dictionary of English v.0.48:

Rancorous \Ran"cor*ous\ (r[a^][ng]"k[~e]r*[u^]s), a. [OF.
   rancuros.]
   Full of rancor; evincing, or caused by, rancor; deeply
   malignant; implacably spiteful or malicious; intensely
   virulent.
   [1913 Webster]

         So flamed his eyes with rage and rancorous ire.
                                                  --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rancorous
    adj 1: showing deep-seated resentment; "preserve...from
           rancourous envy of the rich"- Aldous Huxley

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "rancorous":
   acerb, acerbate, acerbic, acid, acidic, acidulent, acidulous,
   acrid, acrimonious, antagonistic, antipathetic, avenging,
   belligerent, bitter, burning, caustic, choleric, clashing,
   colliding, conflicting, despiteful, embittered, envenomed, evil,
   full of hate, grudgeful, hateful, hostile, implacable,
   irreconcilable, malevolent, malicious, malign, malignant, punitive,
   punitory, quarrelsome, rankled, repugnant, resentful, resenting,
   retaliatory, revanchist, revengeful, set against, sore, spiteful,
   splenetic, stewing, unappeasable, venenate, vengeful, venomous,
   vicious, vindicatory, vindictive, virulent, vitriolic, wicked