verb intrans
said of a wound: to generate pus
said of food, etc: to putrefy or rot
said of a feeling, etc: to rankle
It's the things I might have said that fester — Clemence Dane
said of a problem, situation, etc: to become worse; to intensify
to make (a wound) inflamed
[Middle English festren, from fester, festre suppurating sore: see fester2]