Rancorous means deeply bitter and full of long-lasting anger or resentment. When a disagreement becomes rancorous, it goes beyond a simple argument into something darker, where people nurse grudges and harbor ill will toward each other.

You might see rancorous feelings develop when two former friends have a falling-out and can't let go of their hurt and anger, even years later. A rancorous debate is poisoned by bitterness, where people seem more interested in attacking each other than solving problems. Historical rivals sometimes develop rancorous relationships that last for generations, with each side remembering old wrongs and refusing to forgive.

When a classroom argument becomes rancorous, it means students actively dislike each other and hold onto angry feelings long after the original problem. Rancorous feelings are exhausting to carry around. They're different from momentary anger, which can fade quickly, or healthy disagreement, which can actually help people understand each other better.