To fester means to grow and spread, not in a good way. It can refer to physical wounds or sores that get worse and have liquid or pus oozing out, or to emotional wounds that can have a decaying effect on friendships and the heart. 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. If an argument or bad feeling festers, it continues so that feelings of hate or lack of satisfaction increase.