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.
   [1913 Webster]

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.
      [1913 Webster]

            Unkindness may give a wound that shall bleed and
            smart, but it is treachery that makes it fester.
                                                  --South.
      [1913 Webster]

            Hatred . . . festered in the hearts of the children
            of the soil.                          --Macaulay.
      [1913 Webster]

   2. To be inflamed; to grow virulent, or malignant; to grow in
      intensity; to rankle.
      [1913 Webster]

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.
      [1913 Webster]

            The fester of the chain their necks.  --I. Taylor.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "fester":
   abscess, afflict, agonize, ail, aposteme, bed sore, bite, blain,
   bleb, blister, boil, break up, bubo, bulla, bunion, burn, canker,
   canker sore, carbuncle, chafe, chancre, chancroid, chilblain,
   cold sore, convulse, corrupt, crucify, crumble, crumble into dust,
   cut, decay, decompose, disintegrate, distress, eschar, excruciate,
   fall into decay, fall to pieces, felon, festering, fever blister,
   fistula, fret, furuncle, furunculus, gall, gangrene, gathering,
   give pain, gnaw, go bad, go to pieces, grate, grind, gripe,
   gumboil, harrow, hemorrhoids, hurt, inflame, inflict pain,
   irritate, kibe, kill by inches, lacerate, lesion, martyr,
   martyrize, matter, mildew, mold, molder, mortify, necrose, nip,
   ooze, pain, papula, papule, paronychia, parulis, petechia, pierce,
   piles, pimple, pinch, pock, polyp, prick, prolong the agony,
   pustule, put to torture, putrefy, putresce, rack, rankle, rasp,
   ripen, rising, rot, rub, run, scab, smoulder, soft chancre, sore,
   sphacelate, spoil, stab, stigma, sting, sty, suppurate,
   suppuration, swelling, torment, torture, tubercle, tweak, twist,
   ulcer, ulcerate, ulceration, wale, weep, welt, wheal, whelk,
   whitlow, wound, wring