The Collaborative International Dictionary of English v.0.48:

Job \Job\ (j[o^]b), n. [Prov. E. job, gob, n., a small piece of
   wood, v., to stab, strike; cf. E. gob, gobbet; perh.
   influenced by E. chop to cut off, to mince. See Gob.]
   [1913 Webster]
   1. A sudden thrust or stab; a jab.
      [1913 Webster]

   2. A piece of chance or occasional work; any definite work
      undertaken in gross for a fixed price; as, he did the job
      for a thousand dollars.
      [1913 Webster]

   3. A public transaction done for private profit; something
      performed ostensibly as a part of official duty, but
      really for private gain; a corrupt official business.
      [1913 Webster]

   4. Any affair or event which affects one, whether fortunately
      or unfortunately. [Colloq.]
      [1913 Webster]

   5. A situation or opportunity of work; as, he lost his job.
      [Colloq.]
      [1913 Webster]

   6. A task, or the execution of a task; as, Michelangelo did a
      great job on the David statue.
      [PJC]

   7. (Computers) A task or coordinated set of tasks for a
      multitasking computer, submitted for processing as a
      single unit, usually for execution in background. See job
      control language.
      [PJC]

   Note: Job is used adjectively to signify doing jobs, used for
         jobs, or let on hire to do jobs; as, job printer; job
         master; job horse; job wagon, etc.
         [1913 Webster]

   By the job, at a stipulated sum for the work, or for each
      piece of work done; -- distinguished from time work; as,
      the house was built by the job.

   Job lot, a quantity of goods, usually miscellaneous, sold
      out of the regular course of trade, at a certain price for
      the whole; as, these articles were included in a job lot.


   Job master, one who lest out horses and carriages for hire,
      as for family use. [Eng.]

   Job printer, one who does miscellaneous printing, esp.
      circulars, cards, billheads, etc.

   Odd job, miscellaneous work of a petty kind; occasional
      work, of various kinds, or for various people.

   to do a job on, to harm badly or destroy. [slang]

   on the job, alert; performing a responsibility well.
      [slang]
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

on the job
    adj 1: actively engaged in paid work; "the working population";
           "the ratio of working men to unemployed"; "a working
           mother"; "robots can be on the job day and night" [syn:
           working(a), on the job(p)]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "on the job":
   advertent, agile, agog, alert, alive, all ears, all eyes,
   assiduous, at it, at work, attentive, awake, aware, bright, busy,
   careful, concentrated, conscious, diligent, earnest, employed,
   engaged, finical, finicking, finicky, full of business, hard at it,
   hard at work, heedful, in harness, intense, intent, intentive,
   keen, meticulous, mindful, nice, niggling, nimble, observant,
   observing, occupied, on duty, on the, on the alert, on the ball,
   on the go, on the hop, on the jump, on the move, on the run,
   open-eared, open-eyed, openmouthed, prompt, qui vive, quick, ready,
   regardful, sharp, sleepless, smart, tied up, unblinking, unnodding,
   unsleeping, unwinking, wakeful, watchful, wide-awake, working