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.
late 14c., "hard, unbreakable," from adamant (n.). The figurative sense of "unshakeable" (in belief, etc.) is by 1670s. Related: Adamantly; adamance.
"brief, familiar proverb," 1540s, French adage (16c.), from Latin adagium "adage, proverb," apparently a collateral form of adagio, from ad "to" (see ad-) + *agi-, root of aio "I say," which is perhaps cognate with Armenian ar-ac "proverb," asem "to say." But some find this unlikely and suggest the second element might be related to agein "set in motion, drive, urge" (from PIE root *ag- "to drive, draw out or forth, move"). Related: Adagial.
"a very hard stone," mid-14c., adamant, adamaunt, from Old French adamant "diamond; magnet" or directly from Latin adamantem (nominative adamas) "adamant, hardest iron, steel," also used figuratively, of character, from Greek adamas (genitive adamantos), the name of a hypothetical hardest material.
It is a noun use of an adjective meaning "unbreakable, inflexible," which was metaphoric of anything unalterable (such as Hades) and is of uncertain origin. It is perhaps literally "invincible, indomitable," from a- "not" (see a- (3)) + daman "to conquer, to tame," from PIE root *deme- "to constrain, force, break (horses)," for which see tame (adj.). "But semantically, the etymology is rather strange," according to Beekes, who suggests it might be a foreign word altered in Greek by folk etymology, and compares Akkadian (Semitic) adamu.
Applied in antiquity to a metal resembling gold (Plato), white sapphire (Pliny), magnet (Ovid, perhaps through confusion with Latin adamare "to love passionately"), steel, emery stone, and especially diamond, which is a variant of this word. "The name has thus always been of indefinite and fluctuating sense" [Century Dictionary]. The word had been in Old English as aðamans, but the modern word is a re-borrowing.
1801, "one who adapts (something to something else)," agent noun from adapt. From 1808 as "mechanical means of adapting objects so they fit or work together" (originally of chemistry apparatus); electrical engineering sense is by 1907.