1550s, in phrase jobbe of worke "piece of work" (contrasted with continuous labor), perhaps a variant of gobbe "mass, lump" (c.1400; see gob). Sense of "work done for pay" first recorded 1650s. Slang meaning "specimen, thing, person" is from 1927. The verb is attested from 1660s. 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. (1) A low mean lucrative busy affair. (2) Petty, piddling work; a piece of chance work. [Johnson's Dictionary]