blow-job (n.)

also blowjob, "act of fellatio," 1961, from blow + job (n.). Exactly which blow is meant is the subject of some debate; the word might have begun as a euphemism for suck (thus from blow (v.1)), or it might refer to the explosive climax of an orgasm (thus blow (v.2)). The oldest verbal form appears to be blow (someone) off (1933), a phrase originally among prostitutes.

Unlike much sex slang, its date of origin probably is pretty close to the date it first is attested in print: as recently as the early 1950s, military pilots could innocently talk of their jet planes as blow jobs according to the "Thesaurus of American Slang."