1550s, in phrase jobbe of worke "piece of work" (contrasted with continuous labor), of uncertain origin, perhaps a variant of gobbe "mass, lump" (c.1400; see gob) via sense of "a cart-load." Sense of "work done for pay" first recorded 1650s. Thieves' slang sense of "theft, robbery, a planned crime" is from 1722. Printing sense is from 1795. Slang meaning "specimen, thing, person" is from 1927.
On the job "hard at work" is from 1882. Job lot is from obsolete sense of "cartload, lump," which might also ultimately be from gob. Job security attested by 1954; job description by 1920; job-sharing by 1972.