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.
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   3. (Hort.) A disease incident to trees, causing the bark to
      rot and fall off.
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   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.
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   5. A kind of wild, worthless rose; the dog-rose.
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            To put down Richard, that sweet lovely rose.
            And plant this thorm, this canker, Bolingbroke.
                                                  --Shak.
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   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.
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   2. To infect or pollute; to corrupt. --Addison.
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            A tithe purloined cankers the whole estate.
                                                  --Herbert.
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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.
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   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.
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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