fes·ter / ˈfestər/ • v. [intr.] (of a wound or sore) become septic; suppurate: [as adj.] (festering) a festering abscess. ∎ (of food or garbage) become rotten and offensive to the senses: a gully full of garbage that festered in the sun. ∎ (of a negative feeling or a problem) become worse or more intense, esp. through neglect or indifference: anger which festers and grows in his heart. ∎ (of a person) undergo physical and mental deterioration in isolated inactivity: I might be festering in jail now.
So vb. XIV. f. the sb. or OF. festrir.