Deep-rooted and bitter ill feeling; resentment or animosity, esp. of long standing; an instance of this.

Bitterness; the quality of being like nitre, esp. in respect of its bitterness.

bitter feeling.

Bitter feelings or sentiments; acrimony; resentment; (also) the quality of being bitter to the mind or feelings; distress; anguish. Somewhat rare…

Gall, bitterness; (hence) animosity, rancour.

Scottish. Bitter malice; rancour; an instance of this. Obsolete.

bitter anger.

= virulence, n. 1.

Something which rankles; a rankling thought or feeling. Also as a mass noun: rankling, bitterness.

The action of embittering; the state of being embittered.

Extreme acrimony or bitterness of temper or speech; violent malignity or rancour.

Acrimony, exasperated condition. Obsolete.

Virulence.