chemotherapy n.

before vowels chem-, word-forming element denoting "relation to chemical action or chemicals," from combining form of chemical (adj.), used to form scientific compound words from c. 1900. In 19c., chemico- was used.

1846, "medical treatment of disease," from Modern Latin therapia, from Greek therapeia "curing, healing, service done to the sick; a waiting on, service," from therapeuein "to cure, treat medically," literally "attend, do service, take care of" (see therapeutic).