do duty, to serve the same function; substitute for: bookcases that do duty as room dividers.
15.
off duty, not at one's post or work; at liberty: They spent their days off duty in hiking and fishing.
16.
on duty, at one's post or work; occupied; engaged: He was suspended from the force for being drunk while on duty.
Origin: 1250–1300;Middle Englishdu(e)te < Anglo-Frenchduete. See due, -ty2
Synonyms 1. Duty, obligation refer to what one feels bound to do. Duty is what one performs, or avoids doing, in fulfillment of the permanent dictates of conscience, piety, right, or law: duty to one's country; one's duty to tell the truth, to raise children properly. An obligation is what one is bound to do to fulfill the dictates of usage, custom, or propriety, and to carry out a particular, specific, and often personal promise or agreement: financial obligations.3. responsibility, business. 4. deference.
c.1300, from Anglo-Fr. duete, from O.Fr. deu "due, owed," from V.L. *debutus, from L. debitus, pp. of debere "to owe." Related: Duties. The sense of "tax or fee in imports, exports, etc." is from late 15c.; duty-free as a noun is attested from 1958.