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.]
      [1913 Webster]

            When thou receivest money for thy labor or ware,
            thou receivest thy duty.              --Tyndale.
      [1913 Webster]

   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.
      [1913 Webster]

            Forgetting his duty toward God, his sovereign lord,
            and his country.                      --Hallam.
      [1913 Webster]

   3. Hence, any assigned service or business; as, the duties of
      a policeman, or a soldier; to be on duty.
      [1913 Webster]

            With records sweet of duties done.    --Keble.
      [1913 Webster]

            To employ him on the hardest and most imperative
            duty.                                 --Hallam.
      [1913 Webster]

            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.
      [1913 Webster]

   4. Specifically, obedience or submission due to parents and
      superiors. --Shak.
      [1913 Webster]

   5. Respect; reverence; regard; act of respect; homage. "My
      duty to you." --Shak.
      [1913 Webster]

   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).
      [1913 Webster]

   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.
      [1913 Webster]

   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.]
         [1913 Webster]

   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.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DUTIES. In its most enlarged sense, this word is nearly equivalent to taxes,
embracing all impositions or charges levied on persons or things; in its
more restrained sense, it is often used as equivalent to customs, (q.v.) or
imposts. (q.v.) Story, Const. Sec. 949. Vide, for the rate of duties
payable on goods and merchandise, Gord. Dig. B. 7, t. 1, c. 1; Story's L. U.
S. Index, h.t.