Old English ahte "owned, possessed," past tense of agan "to own, possess; owe" (see owe). As a past tense of owe, it shared in that word's evolution and meant at times in Middle English "possessed" and "under obligation to pay." It has been detached from owe since 17c., though he aught me ten pounds is recorded as active in East Anglian dialect c. 1825. As an auxiliary verb expressing duty or moral obligation (the main modern use, attested from late 12c.), it represents the past subjunctive.

Ought, Should. Ought is the stronger, expressing especially obligations of duty, with some weaker use in expressing interest or necessity: as, you ought to know, if any one does. Should sometimes expresses duty: as, we should be careful of others' feelings; but generally expresses propriety, expediency, etc.: as, we should dot our i's and cross our t's. [Century Dictionary, 1895] 

naval officer, c. 1600, originally so called because he was stationed amidships when on duty (see amid). Midships as short for amidships is by 1620s. Midship as "the middle of a ship or boat" is from 1550s.

"fact or condition of being responsible, accountable, or answerable," 1787, from responsible + -ity. Meaning "that for which one is responsible; a trust, duty, etc.," is from 1796. In 19c. often with of; in 20c. often with for. Related: Responsibilities.

"binding in law or conscience, imposing duty, requiring performance of or forbearance from some act," c. 1400, obligatorie, from Old French obligatoire "creating an obligation, obligatory," and directly from Late Latin obligatorius "binding," from obligat-, past-participle stem of obligare "to bind, bind up, bandage," figuratively "put under obligation" (see oblige).

"writ from a superior court to an inferior court or officer specifying that something be done by the persons addressed, as being within their office or duty," 1530s (late 14c. in Anglo-French), from Latin mandamus "we order" (opening word of the writ), first person plural present indicative of mandare "to order" (see mandate (n.)). "Its use is generally confined to cases of complaint by some person having an interest in the performance of a public duty, when effectual relief against its neglect cannot be had in the course of an ordinary action" [Century Dictionary]. 

"to perform the duty of a priest," 1630s, from Medieval Latin officiatus, present participle of officiare "perform religious services," from Latin officium "a service" (in Medieval Latin, "church service"); see office. The earlier verb in English was simply office (mid-15c.). Related: Officiated; officiating.

"of, pertaining to, or characterized by selection;" hence "using that which is choice," 1620s; see select (adj.) + -ive. Related: Selectively; selectivity; selectiveness. Selective service as a military drafting system is from 1917, American English; hence selectee "person chosen for military duty" (by 1940).