Words related to a-
late 14c. in Latin form abyssus, "depths of the earth or sea; primordial chaos;" early 14c. as abime "depths of the earth or sea; bottomless pit, Hell" (via Old French; see abysm). Both are from Late Latin abyssus "bottomless pit," from Greek abyssos (limnē) "bottomless (pool)," from abyssos "bottomless, unfathomed," hence, generally, "enormous, unfathomable," also as a noun, he abyssos "the great depth, the underworld, the bottomless pit." This is a compound of a- "without" (see a- (3)) + byssos "bottom," a word of uncertain origin possibly related to bathos "depth" [Liddell & Scott]. Watkins suggests a connection with the root of bottom (n.); Beekes suggests it is pre-Greek.
The current form in English is a 16c. partial re-Latinization. Greek abyssos was used in Septuagint to translate Hebrew tehom "original chaos" and was used in the New Testament for "Hell." OED notes, "the word has had five variants, abime, abysm, abysmus, abyssus, abyss; of which abyss remains as the ordinary form, and abysm as archaic or poetic." In reference to a seemingly bottomless gulf from 1630s. Old English glossed Latin abyssum with deagenesse, which is related to deagol "secret, hidden; dark, obscure."
Principally in botany and zoology, but also "without a leader" (1751). Acephali as the name of a fabulous race of men with no heads, said by ancient writers to inhabit part of Africa, is attested from c. 1600, from Late Latin plural of acephalus, from Greek akephalos; the name also appears in Church history in reference to sects that refused to have priests or bishops (1620s). Related: Acephalian (1580s); acephalic (1650s).
"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.
"without transference, impossible (to heat)," 1838, with -ic + Greek adiabatos "not to be passed" (of rivers, etc.), from a- "not" (see a- (3)) + diabatos "to be crossed or passed, fordable," from dia "through" (see dia-) + batos "passable," from bainein "to go, walk, step" (from PIE root *gwa- "to go, come"). In thermodynamics, of a change in volume without change in heat.