The Collaborative International Dictionary of English v.0.48:

Canker \Can"ker\ (k[a^][ng]"k[~e]r), n. [OE. canker, cancre, AS.
   cancer (akin to D. kanker, OHG chanchar.), fr. L. cancer a
   cancer; or if a native word, cf. Gr. ? excrescence on tree, ?
   gangrene. Cf. also OF. cancre, F. chancere, fr. L. cancer.
   See cancer, and cf. Chancre.]
   [1913 Webster]
   1. A corroding or sloughing ulcer; esp. a spreading
      gangrenous ulcer or collection of ulcers in or about the
      mouth; -- called also water canker, canker of the
      mouth, and noma.
      [1913 Webster]

   2. Anything which corrodes, corrupts, or destroy.
      [1913 Webster]

            The cankers of envy and faction.      --Temple.
      [1913 Webster]

   3. (Hort.) A disease incident to trees, causing the bark to
      rot and fall off.
      [1913 Webster]

   4. (Far.) An obstinate and often incurable disease of a
      horse's foot, characterized by separation of the horny
      portion and the development of fungoid growths; -- usually
      resulting from neglected thrush.
      [1913 Webster]

   5. A kind of wild, worthless rose; the dog-rose.
      [1913 Webster]

            To put down Richard, that sweet lovely rose.
            And plant this thorm, this canker, Bolingbroke.
                                                  --Shak.
      [1913 Webster]

   Black canker. See under Black.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Canker \Can"ker\ (k[a^][ng]"k[~e]r), v. t. [imp. & p. p.
   Cankered (-k[~e]rd); p. pr. & vb. n. Cankering.]
   1. To affect as a canker; to eat away; to corrode; to
      consume.
      [1913 Webster]

            No lapse of moons can canker Love.    --Tennyson.
      [1913 Webster]

   2. To infect or pollute; to corrupt. --Addison.
      [1913 Webster]

            A tithe purloined cankers the whole estate.
                                                  --Herbert.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Canker \Can"ker\, v. i.
   1. To waste away, grow rusty, or be oxidized, as a mineral.
      [Obs.]
      [1913 Webster]

            Silvering will sully and canker more than gliding.
                                                  --Bacom.
      [1913 Webster]

   2. To be or become diseased, or as if diseased, with canker;
      to grow corrupt; to become venomous.
      [1913 Webster]

            Deceit and cankered malice.           --Dryden.
      [1913 Webster]

            As with age his body uglier grows,
            So his mind cankers.                  --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

canker
    n 1: a fungal disease of woody plants that causes localized
         damage to the bark
    2: an ulceration (especially of the lips or lining of the mouth)
       [syn: canker, canker sore]
    3: a pernicious and malign influence that is hard to get rid of;
       "racism is a pestilence at the heart of the nation";
       "according to him, I was the canker in their midst" [syn:
       pestilence, canker]
    v 1: become infected with a canker
    2: infect with a canker

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "canker":
   abscess, adulterate, alloy, aposteme, bed sore, blain, blast, bleb,
   blight, blister, boil, break up, bubo, bulla, bunion, cancer,
   canker sore, carbuncle, chancre, chancroid, cheapen, chilblain,
   coarsen, cold sore, confound, contaminate, corrode, corrupt,
   crumble, crumble into dust, debase, debauch, decay, decompose,
   defile, deflower, degenerate, degrade, demoralize, denature,
   deprave, desecrate, despoil, devalue, disintegrate, distort,
   dry rot, eat, eat away, eat into, erode, eschar, fall into decay,
   fall to pieces, felon, fester, festering, fever blister, fistula,
   fungus, furuncle, furunculus, gangrene, gathering, gnaw, go bad,
   go to pieces, gumboil, hemorrhoids, infect, kibe, lesion, mildew,
   misuse, mold, molder, mortify, moth, moth and rust, must, necrose,
   nibble away, oxidize, papula, papule, paronychia, parulis, pervert,
   pest, petechia, piles, pimple, pock, poison, pollute, polyp,
   prostitute, pustule, putrefy, putresce, rankle, ravage, ravish,
   rising, rot, rust, scab, smut, soft chancre, sore, sphacelate,
   spoil, stain, stigma, sty, suppurate, suppuration, swelling, taint,
   tubercle, twist, ulcer, ulcerate, ulceration, violate, vitiate,
   vulgarize, wale, warp, welt, wheal, whelk, whitlow, worm, wound



Easton's 1897 Bible Dictionary:

Canker
   a gangrene or mortification which gradually spreads over the
   whole body (2 Tim. 2:17). In James 5:3 "cankered" means "rusted"
   (R.V.) or tarnished.