What is "canker of the mouth"

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.