To fester is to grow and spread, not in a good way. When a cut gets infected it starts to fester and smell bad. Emotional wounds stink too, like when you hold on to anger or pain until it starts to fester and explodes.
Fester is a verb describing what happens to a wound or a sore that gets worse and has liquid, or pus, oozing out. Infections cause cuts, broken bones, and diseases to fester. Dead bodies can fester too — as they decompose. Things that fester have a decaying odor, and bad feelings can have a decaying effect on friendships and the heart. Letting bitter emotions fester often leads to their getting worse.