du·ty 
(dtē, dy-)
(dtē, dy-)n. pl. du·ties
Idioms: 1.
a. An act or a course of action that is required of one by position, social custom, law, or religion: the duties of being a critical care nurse.
b. Required action or service: jury duty; beyond the call of duty. See Synonyms at function.
c. Active military service: a tour of duty.
2.
a. Moral or legal obligation: It is your duty to tell the truth.
b. The compulsion felt to meet such obligation: acting out of duty.
3. A tax charged by a government, especially on imports.
4.
a. The application of something for a purpose; use: The dining room table also does duty as a desk.
b. A measure of efficiency expressed as the amount of work done per unit of energy used.
5. The total volume of water required to irrigate a given area in order to cultivate a specific crop until harvest.
duty bound
Obliged: You are duty bound to help your little sister and brother.
off duty
Not engaged in or responsible for assigned work.
on duty
Engaged in or responsible for assigned work.
[Middle English duete, from Anglo-Norman, from due, variant of Old French deu, due; see DUE.]