late 14c., "vapor, odorous vapor; exhalation," from Old French fum "smoke, steam, vapor, breath, aroma, scent" (12c.), from Latin fumus "smoke, steam, fume, old flavor" (source also of Italian fumo, Spanish humo), from PIE root *dheu- (1) "dust, vapor, smoke."
In old medicine, an "exhalation" of the body that produces emotions, dreams, sloth, etc; later especially of smokes or vapors that go to the head and affect the senses with a narcotic or stifling quality.
c. 1400, "to fumigate" (transitive), from Old French fumer "to smoke, burn" (12c.), from Latin fumare "to smoke, steam," from fumus "smoke, steam, fume" (from PIE root *dheu- (1) "dust, vapor, smoke"). Intransitive meaning "throw off smoke, emit vapor" is from 1530s; the figurative sense "show anger, be irritated" is slightly earlier (1520s). Related: Fumed; fumes; fuming.