Source : Webster's Revised Unabridged Dictionary (1913)

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}.]
   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}.

   2. Anything which corrodes, corrupts, or destroy.

            The cankers of envy and faction.      --Temple.

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

   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.

   5. A kind of wild, worthless rose; the dog-rose.

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

   {Black canker}. See under {Black}.

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.

            No lapse of moons can canker Love.    --Tennyson.

   2. To infect or pollute; to corrupt. --Addison.

            A tithe purloined cankers the whole estate.
                                                  --Herbert.

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

            Silvering will sully and canker more than gliding.
                                                  --Bacom.

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

            Deceit and cankered malice.           --Dryden.

            As with age his body uglier grows, So his mind
            cankers.                              --Shak.

Source : WordNet®

canker
     n : an ulceration (especially of the lips or lining of the
         mouth)

canker
     v 1: become infected with a canker
     2: infect with a canker