noun

• (pathology, obsolete) A fistula.
• (pathology) A sore or an ulcer of the skin.
  "The larger the Spider, the warmer the climate or season of the year, and the more susceptible the wounded individual, so much worse will the effects be; and it is no therefore no wonder that people who would have a fester from a simple prick with a needle, should feel more violent effects from the bite of a Spider."

verb

• (intransitive) To become septic; to become rotten.
  "and she for the despyte of her sones dethe wrought by her subtyl craftes that syr Vrre shold neuer be hole but euer his woundes shold one tyme feyster & another tyme blede "and she, for the despite of her son’s death, wrought by her subtle crafts that Sir Urre should never be whole, but ever his wounds should one time fester and another time bleed""
• (intransitive) To worsen, especially due to lack of attention.
  "Deal with the problem immediately; do not let it fester."
• (transitive) To cause to fester or rankle.
  "For which I burnt in inward sweltring hate, / And festred rankling malice in my breast, / Till I might belke revenge upon his eyes: […]"