the.com/rancor

means A deep, bitter resentment held onto for a long time — hatred with a grudge attached.

from From Old French rancor, from Latin rancor meaning 'rankness' or 'grudge,' tied to the verb rancere, 'to be rotten or stink.' The same root gives us 'rancid' — so rancor is etymologically the smell of something gone bad, which fits a grudge that's been left out too long.