The Collaborative International Dictionary of English v.0.48:

Fester \Fes`ter\, v. t.
   To cause to fester or rankle.
   [1913 Webster]

         For which I burnt in inward, swelt'ring hate,
         And festered ranking malice in my breast. --Marston.
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The Collaborative International Dictionary of English v.0.48:

Fester \Fes"ter\, v. i. [imp. & p. p. Festered; p. pr. & vb.
   n. Festering.] [OE. festern, fr. fester, n.; or fr. OF.
   festrir, fr. festre, n. See Fester, n.]
   1. To generate pus; to become imflamed and suppurate; as, a
      sore or a wound festers.
      [1913 Webster]

            Wounds immedicable
            Rankle, and fester, and gangrene.     --Milton.
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            Unkindness may give a wound that shall bleed and
            smart, but it is treachery that makes it fester.
                                                  --South.
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            Hatred . . . festered in the hearts of the children
            of the soil.                          --Macaulay.
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   2. To be inflamed; to grow virulent, or malignant; to grow in
      intensity; to rankle.
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The Collaborative International Dictionary of English v.0.48:

Fester \Fes"ter\, n. [OF. festre, L. fistula a sort of ulcer.
   Cf. Fistula.]
   1. A small sore which becomes inflamed and discharges corrupt
      matter; a pustule.
      [1913 Webster]

   2. A festering or rankling.
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            The fester of the chain their necks.  --I. Taylor.
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WordNet (r) 3.0 (2006):

fester
    n 1: a sore that has become inflamed and formed pus [syn:
         fester, suppurating sore]
    v 1: ripen and generate pus; "her wounds are festering" [syn:
         fester, maturate, suppurate]