Rancor

/ˈræŋ.kɚ/ noun medium

A deep, long-lasting feeling of bitterness or resentment toward someone.

Years after the lawsuit ended, the two former partners still spoke to each other with rancor.

Memory Trick

Rancor sounds like 'rank odor' — a grudge left to sit so long that it has gone rank and sours the whole relationship.