Fester (verb)

1. To form pus; to generate an abscess.
2. To become infected and filled with pus; to develop a sore.

Origin:

From middle english festren, from old english festerian "putrefy, become putrid", from proto-germanic *fastaz "fast".

Examples:

1. The wound festered and became infected.
2. The wound festered and had to be drained.
3. The resentment festered and turned into anger.
4. The grudge festered and caused a rift in the relationship.
5. The issue festered for months before it was finally addressed.