On duty

from The Collaborative International Dictionary of English v.0.48
Duty \Du"ty\, n.; pl. {Duties}. [From {Due}.]
   1. That which is due; payment. [Obs. as signifying a material
      thing.]
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            When thou receivest money for thy labor or ware,
            thou receivest thy duty.              --Tyndale.
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   2. That which a person is bound by moral obligation to do, or
      refrain from doing; that which one ought to do; service
      morally obligatory.
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            Forgetting his duty toward God, his sovereign lord,
            and his country.                      --Hallam.
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   3. Hence, any assigned service or business; as, the duties of
      a policeman, or a soldier; to be on duty.
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            With records sweet of duties done.    --Keble.
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            To employ him on the hardest and most imperative
            duty.                                 --Hallam.
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            Duty is a graver term than obligation. A duty hardly
            exists to do trivial things; but there may be an
            obligation to do them.                --C. J. Smith.
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   4. Specifically, obedience or submission due to parents and
      superiors. --Shak.
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   5. Respect; reverence; regard; act of respect; homage. "My
      duty to you." --Shak.
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   6. (Engin.) The efficiency of an engine, especially a steam
      pumping engine, as measured by work done by a certain
      quantity of fuel; usually, the number of pounds of water
      lifted one foot by one bushel of coal (94 lbs. old
      standard), or by 1 cwt. (112 lbs., England, or 100 lbs.,
      United States).
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   7. (Com.) Tax, toll, impost, or customs; excise; any sum of
      money required by government to be paid on the
      importation, exportation, or consumption of goods.
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   Note: An impost on land or other real estate, and on the
         stock of farmers, is not called a duty, but a direct
         tax. [U.S.]
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   {Ad valorem duty}, a duty which is graded according to the
      cost, or market value, of the article taxed. See {Ad
      valorem}.

   {Specific duty}, a duty of a specific sum assessed on an
      article without reference to its value or market.

   {On duty}, actually engaged in the performance of one's
      assigned task.
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