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.
early 14c., "to carry out, fulfill" (transitive), probably from Old French compli, past participle of complir "to accomplish, fulfill, carry out," from Vulgar Latin *complire, from Latin complere "to fill up," transferred to "fulfill, finish (a task)," from com-, here probably as an intensive prefix (see com-), + plere "to fill" (from PIE root *pele- (1) "to fill").
Intransitive sense of "to consent, act in accordance with another's will or desire" is attested from c. 1600 and might have been influenced by ply (v.2), or perhaps it is a reintroduction from Italian, where complire had come to mean "satisfy by 'filling up' the forms of courtesy" (compare compliment (n.)).