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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WordNet (r) 3.0 (2006):

duty
    n 1: the social force that binds you to the courses of action
         demanded by that force; "we must instill a sense of duty in
         our children"; "every right implies a responsibility; every
         opportunity, an obligation; every possession, a duty"- John
         D.Rockefeller Jr [syn: duty, responsibility,
         obligation]
    2: work that you are obliged to perform for moral or legal
       reasons; "the duties of the job"
    3: a government tax on imports or exports; "they signed a treaty
       to lower duties on trade between their countries" [syn:
       duty, tariff]

Moby Thesaurus II by Grady Ward, 1.0:

279 Moby Thesaurus words for "duty":
   Mass, VAT, accountability, active service, ad valorem duty,
   admiration, adoration, alcohol tax, allegiance, amenability,
   amusement tax, answerability, apotheosis, appreciation,
   approbation, approval, assessment, assessment on default,
   assignment, awe, bedtime prayer, bit, blackmail,
   breathless adoration, burden, business, busywork, call, call for,
   calling, camp meeting, capacity, capital gains tax, capitation,
   capitation tax, cargo, ceremonial, ceremony, cess, character,
   chare, charge, chore, church, church service, claim, commission,
   commitment, committal, compline, conscience money, consideration,
   contribution, corporation tax, courtesy, customs, customs duty,
   deadweight, death duty, death tax, deference, deification, demand,
   demand for, devoir, devotions, direct tax, divine service, doomage,
   draft, drain, end use, errand, estate duty, estate tax, esteem,
   estimation, evening devotions, evensong, exaction,
   exaggerated respect, excess profits tax, excise, excise tax,
   exercise, exercises, export tax, extortion, extortionate demand,
   faithfulness, favor, fealty, federal tax, fidelity, fish to fry,
   form, form of worship, formality, formula, formulary, freight,
   function, gabelle, gift tax, goal, graduated taxation,
   great respect, head tax, heavy demand, hero worship, high regard,
   holy rite, homage, homework, honor, idolatry, idolization,
   immediate purpose, import tax, imposition, impost, income tax,
   indent, indirect tax, inheritance tax, insistent demand,
   institution, internal revenue tax, job, job of work, joint return,
   labor, land tax, lauds, levy, liability, liquor tax, liturgy, load,
   local tax, loyalty, luxury tax, make-work, mark, matins,
   matters in hand, meeting, millstone, mission, mode of worship,
   morning devotions, must, mystery, national service, need,
   night song, none, nones, nonnegotiable demand, notice, novena,
   nuisance tax, object, objective, obligation, observance,
   occupation, odd job, office, onus, operation, operational purpose,
   order, order of worship, ordinance, ought, part,
   personal property tax, piece of work, place, poll, poll tax,
   position, practice, praise meeting, prayer, prayer meeting,
   prayers, prescribed form, prestige, prime, prime song,
   progressive tax, project, property tax, property-increment tax,
   protective tariff, province, provincial tax, public worship,
   purpose, rates, regard, requirement, requisition, respect,
   responsibility, revenue tariff, reverence, reverential regard,
   revival, revival meeting, right, rite, ritual, ritual observance,
   rituality, role, rush, rush order, sacrament, sacramental,
   sales tax, salt tax, school tax, selective service,
   separate returns, service, severance tax, sext, single tax,
   solemnity, specific duty, state tax, stint, supertax, surtax,
   target, tariff, tariff duty, task, tax, tax base, tax dodging,
   tax evasion, tax exemption, tax return, tax structure,
   tax withholding, tax-exempt status, taxable income, taxation,
   taxing, telephone tax, tent meeting, things to do, tierce, tithe,
   toll, tribute, ultimate purpose, ultimatum, undersong, use,
   use tax, value added tax, veneration, vesper, vespers, vigils,
   warning, watch meeting, watch night, watch-night service, weight,
   window tax, withholding tax, work, worship



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

DUTY, natural law. A human action which is, exactly conformable to the laws
which require us to obey them.
     2. It differs from a legal obligation, because a duty cannot always be
enforced by the law; it is our duty, for example, to be temperate in eating,
but we are under no legal obligation to be so; we ought to love our
neighbors, but no law obliges us to love them.
     3. Duties may be considered in the relation of man towards God, towards
himself, and towards mankind. 1. We are bound to obey the will of God as far
as we are able to discover it, because he is the sovereign Lord of the
universe who made and governs all things by his almighty power, and infinite
wisdom. The general name of this duty is piety: which consists in
entertaining just opinions concerning him, and partly in such affections
towards him, and such, worship of him, as is suitable to these opinions.
     4.-2. A man has a duty to perform towards himself; he is bound by the
law of nature to protect his life and his limbs; it is his duty, too, to
avoid all intemperance in eating and drinking, and in the unlawful
gratification of all his other appetites.
     5.-3. He has duties to perform towards others. He is bound to do to
others the same justice which he would have a right to expect them to do to
him.



The Devil's Dictionary (1881-1906):

DUTY, n.  That which sternly impels us in the direction of profit,
along the line of desire.

    Sir Lavender Portwine, in favor at court,
    Was wroth at his master, who'd kissed Lady Port.
    His anger provoked him to take the king's head,
    But duty prevailed, and he took the king's bread,
            Instead.
                                                                  G.J.