To ulcerate is to fester, or to develop into a terrible, painful sore. Ouch!
In medical terms, when a wound ulcerates, it doesn't heal, but becomes worse — red, open, painful, and sometimes infected. Skin can ulcerate, resulting in bedsores or canker sores in the mouth, and internal wounds can also ulcerate, forming what's known as an ulcer, or open sore. The Latin root of both words, ulcus, means "sore."