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
When thou receivest money for thy labor or ware,
thou receivest thy duty. --Tyndale.
2. That which a person is bound by moral obligation to do, or
refrain from doing; that which one ought to do; service
Forgetting his duty toward God, his sovereign lord,
and his country. --Hallam.
3. Hence, any assigned service or business; as, the duties of
a policeman, or a soldier; to be on duty.
With records sweet of duties done. --Keble.
To employ him on the hardest and most imperative
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.
4. Specifically, obedience or submission due to parents and
5. Respect; reverence; regard; act of respect; homage. "My
duty to you." --Shak.
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.,
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.
Note: An impost on land or other real estate, and on the
stock of farmers, is not called a duty, but a direct
Ad valorem duty, a duty which is graded according to the
cost, or market value, of the article taxed. See Ad
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
Moby Thesaurus II by Grady Ward, 1.0:
18 Moby Thesaurus words for "on duty":
at it, at work, busy, employed, engaged, full of business,
hard at it, hard at work, in harness, occupied, on the go,
on the hop, on the job, on the jump, on the move, on the run,
tied up, working