early 15c. (implied in adapted) "to fit (something, for some purpose)," from Old French adapter (14c.), from Latin adaptare "adjust, fit to," from ad "to" (see ad-) + aptare "to join," from aptus "fitted" (see apt). Intransitive meaning "to undergo modification so as to fit new circumstances" is from 1956. Related: Adapting.

c. 1600, "action of adapting (something to something else)," from French adaptation, from Late Latin adaptationem (nominative adaptatio), noun of action from past-participle stem of adaptare "to adjust," from ad "to" (see ad-) + aptare "to join," from aptus "fitted" (see apt).

The meaning "condition of being adapted, state of being fitted to circumstances or relations" is from 1670s. The sense of "modification of a thing to suit new conditions" is from 1790. The biological sense of "variations in a living thing to suit changed conditions" is by 1859, in Darwin's writings.

1690s, "completely skilled, well-versed," from Latin adeptus "having reached or attained," past participle of adipisci "to come up with, arrive at," figuratively "to attain to, acquire," from ad "to" (see ad-) + apisci "to grasp, attain" (related to aptus "fitted," from PIE root *ap- (1) "to take, reach," for which see apt). Related: Adeptly; adeptness.

"the tip, point, or summit" of anything, c. 1600, from Latin apex "summit, peak, tip, top, extreme end;" which is plausibly related to apere "to fasten, fix," hence "the tip of anything" (one of the meanings of apex in Latin was "small rod at the top of the flamen's cap"), and thus ultimately from PIE *ap- (1) "to take, reach" (see apt). But if the original notion was "point," not "top," it might go another way. Proper plural is apices.

early 15c., "tendency, likelihood," from Late Latin aptitudo (genitive aptitudinis) "fitness," noun of quality from Latin aptus "joined, fitted" (see apt). The meaning "natural capacity to learn" is by 1540s; that of "state or quality of being fit (for a purpose or position)" is from 1640s. Related: Aptitudinal. A doublet of attitude.

early 15c., "by natural means;" 1540s, "in a suitable manner," from apt + -ly (2).

"state or quality of being apt" in any sense, 1530s, from apt + -ness.

early 15c., copulaten, "to join" (transitive), from Latin copulatus, past participle of copulare "join together, couple, bind, link, unite," from copula "band, tie, link," from PIE *ko-ap-, from *ko(m)- "together" (see com-) + *ap- (1) "to take, reach" (see apt). The intransitive sense of "unite sexually" is attested from 1630s. Related: Copulated; copulating.

"ill-suited to the purpose or occasion," 1734, from in- (1) "not, opposite of" + apt. Related: Inaptly; inaptness. Compare inept.

c. 1600, "not fit or suitable, inapt," also "absurd, foolish," from French inepte "incapable" (14c.) or directly from Latin ineptus "unsuitable, improper, impertinent; absurd, awkward, silly, tactless," from in- "not, opposite of" (see in- (1)) + aptus "apt" (see apt). Related: Ineptly; ineptness.