late Old English cancer "spreading ulcer, cancerous tumor," from Latin cancer "malignant tumor," literally "crab" (see cancer, which is its doublet). The form was influenced in Middle English by Old North French cancre "canker, sore, abscess" (Old French chancre, Modern French chancre).
The word was the common one for "cancer" until c. 1700, but since the reintroduction of cancer in a more scientific sense it has tended to be restricted to gangrenous sores of the mouth. Also used since 15c. of caterpillars and insect larvae that eat plant buds and leaves. As a verb, "to corrode, corrupt," from late 14c. Related: Cankered; cankerous.