Rancor
Etymology
From Latin "rancor" (rancidity, a stinking), from "rancēre" (to be rank or rotten). It shares its root with "rancid": the word likens a nursed grudge to food gone off — resentment left until it stinks.
noun · /RANG-kur/
Long-held, bitter resentment or ill will.
What it names
Names bitterness that has aged — a grievance kept until it sours.
Example
Decades on, the merger was still discussed with a rancor that startled younger staff.
Related idea
Resentment is the fresh sense of having been wronged; rancor is resentment that has curdled and settled in — bitter and lasting.