"piece of work; something to be done," 1620s, from phrase jobbe of worke (1550s) "task, piece of work" (contrasted with continuous labor), a word of uncertain origin. Perhaps a variant of gobbe "mass, lump" (c. 1400; see gob) via sense of "a cart-load." The specific sense of "work done for pay" is recorded by 1650s.
job. (1) A low mean lucrative busy affair. (2) Petty, piddling work; a piece of chance work. [Johnson's Dictionary]
The meaning "paid position of employment" is from 1858. Printers' slang sense "piece of work of a miscellaneous class" (posters, handbills, etc.) is from 1795, hence job-type (notably large or ornamental or of exceptional form), job-shop, etc. Job lot (1832) is from an obsolete sense of "cartload, lump," which might be a separate formation from gob.
The very broad general sense of "occurrence, business, state of things" is colloquial from c. 1700. In modern slang or colloquial use, "an example," especially a good one (of the thing indicated), 1927, "a term of wide application" [OED]. Thieves' slang sense of "theft, robbery, a planned crime" is from 1722. Slang meaning "specimen, thing, person" is from 1927.
On the job "hard at work" is from 1882. Job security attested by 1932 (job insecurity by 1936); job description by 1920; job-sharing by 1972. Job-hunter is from 1928. The phrase job of work still appears as late as Trollope (1873).