Acrimonious
Etymology
From Latin "acrimonia" (sharpness, pungency), built on "acer" (sharp, bitter) — the same root behind "acrid" and "acid." The word first named a literal sharpness of taste or smell, then migrated to the figurative sharpness of a temper or a tongue. By the seventeenth century the bitter-flavor sense had largely given way to the bitter-feeling one.
adjective · /ak-ruh-MOH-nee-us/
Marked by sharp, bitter hostility, especially in speech or dispute.
What it names
Reach for it when a disagreement has soured past mere heat into lasting personal rancor.
Example
What began as a routine budget meeting turned acrimonious the moment old grievances resurfaced.
Related idea
Where a debate can be merely heated, an acrimonious one carries lingering personal bitterness — the rancor outlasts the argument itself.