Rancor, n. [Written also rancour.]
Etymology: [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. -- 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.
Shades of meaning
Fernald has no entry of its own for rancor; he lists it under enmity, hatred.
Where he writes about it:
Malice is direct, pressing toward a result; malignity is deep, lingering, and venomous, tho often impotent to act; rancor (akin to rancid) is cherished malignity that has soured and festered and is virulent and implacable. — from hatred