To fester is to grow and spread, not in a good way. It can refer to both physical and emotional wounds. When a wound gets infected, it starts to fester and can generate pus. Emotional wounds can also fester, leading to feelings of hate or lack of satisfaction increasing. Allowing problems to fester can cause them to become worse over time.

It is worth noting that "Uncle Fester" is also a character from the Addams Family franchise. He is a completely hairless, hunched, and barrel-shaped man with dark, sunken eyes and often a deranged smile. He has a strange ability to generate electricity and would often demonstrate this by putting a light bulb in his mouth, which would illuminate, accompanied by a loud, crackling noise.