Merriam-Webster's Collegiate® Dictionary, 11th Edition
FESTER
I. noun
Etymology: Middle English, from Anglo-French festre, from Latin fistula pipe, fistulous ulcer
Date: 14th century
a suppurating sore ; pustule ,
II. verb (festered; festering)
Date: 14th century
intransitive verb to generate pus, putrefy , rot ,
3. to cause increasing poisoning, irritation, or bitterness ; rankle , to undergo or exist in a state of progressive deterioration , transitive verb to make inflamed or corrupt