off-duty (adj.)

by c. 1200 as an emphatic form of Old English of (see of), employed in the adverbial use of that word. The prepositional meaning "away from" and the adjectival sense of "farther" were not firmly fixed in this variant until 17c., but once they were they left the original of with the transferred and weakened senses of the word. Meaning "not working" is from 1861.

Off the cuff "extemporaneously, without preparation" (1938) is from the notion of speaking from notes written in haste on one's shirt cuffs. In reference to clothing, off the rack (adj.) "not tailored, not made to individual requirements, ready-made" is by 1963, on the notion of buying it from the rack of a clothing store; off the record "not to be publicly disclosed" is from 1933; off the wall "crazy" is 1968, probably from the notion of a lunatic "bouncing off the walls" or else in reference to carom shots in squash, handball, etc.

late 14c., duete, "obligatory service, that which ought to be done," also "the force of that which is morally right," from Anglo-French duete, from Old French deu "due, owed," hence "proper, just" (on the notion of "that which one is bound by natural, moral, or legal obligation to do or perform"); from Vulgar Latin *debutus, from Latin debitus, past participle of debere "to owe," originally, "keep something away from someone," from de- "away" (see de-) + habere "to have" (from PIE root *ghabh- "to give or receive"). Related: Duties.

Military sense of "a requisite service" is by 1580s. The sense of "tax or fee on imports, exports, etc." is from late 14c.; hence duty-free (adv.) "free from tax or duty" (1680s), and, as a noun, "duty-free article" (1958), "duty-free shop" (by 1980).