Rancorous (adjective)

1. Feeling or showing a bitter and long-standing resentment.
2. Full of or characterized by deep-seated ill will.

Origin:

Late 16th century: from latin rancorosus, from rancor (see rancor).

Examples:

1. The two families had a rancorous relationship that had lasted for generations.
2. The rancorous arguments between the politicians showed no signs of ending.
3. She had a rancorous hatred for anyone who crossed her.
4. The rancorous exchange between the two lawyers lasted for hours.
5. The rancorous tone of the debate left a bad taste in everyone's mouth.