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.

fester †fistula; ulcer, suppuration. XIII. — OF. festre :- L. fistula.
So vb. XIV. f. the sb. or OF. festrir.

fester (fes-ter) vb. (of superficial wounds) to become inflamed, with the formation of pus.