'canker' originates from Old French (including dialects such as Old North French) words like 'chancre'/'cancra', ultimately from Latin 'cancer', where 'cancer' meant 'crab' and was used for tumors or ulcers.

'canker' changed from Latin 'cancer' into Old French 'chancre' (or similar forms) and entered Middle English as forms such as 'cankor'/'canker', eventually becoming the modern English 'canker'.

Initially it referred to a physical ulcerous growth or tumor ('cancer'/'ulcer'); over time the sense broadened to include plant diseases (lesions, blight) and figurative uses meaning a spreading corruption or moral decay.