c. 1200, "toward the rear," a contraction of Old English on bæc "backward, behind, at or on the back;" see see a- (1) + back (n.). Now surviving mainly in taken aback, which originally was a nautical expression in reference to a vessel's square sails when a sudden change of wind flattens them back against the masts and stops the forward motion (1754). The figurative sense from this, "suddenly or unexpectedly checked or disappointed," is by 1792.

late 14c., used in Revelation ix.11 of "the angel of the bottomless pit," and by Milton of the pit itself, from Hebrew Abhaddon, literally "destruction," from abhadh "he perished." The Greek form was Apollyon.

in civil law, "transfer title of ownership to another," 1550s, from Latin abalienatus, past participle of abalienare "to remove, separate, alienate, make formal transfer of," literally "to convey away," from ab "off, away from" (see ab-) + alienare "to separate" (from alius "another, other, different," from PIE root *al- "beyond"). Related: Abalienated; abalienating.

"act of transferring title of ownership," 1650s, from Latin abalienationem (nominative abalienatio), in law, "transfer of property, sale," noun of action from past-participle stem of abalienare "to separate, transfer the ownership of," from ab "off, away from" (see ab-) + alienare "to separate" (from alius "another, other, different," from PIE root *al- (1) "beyond").

late 14c., "to give up (something) absolutely, relinquish control, give over utterly;" also reflexively, "surrender (oneself), yield (oneself) utterly" (to religion, fornication, etc.), from Old French abandonner "surrender, release; give freely, permit," also reflexive, "devote (oneself)" (12c.).

The Old French word was formed from the adverbial phrase à bandon "at will, at discretion," from à "at, to" (from Latin ad; see ad-) + bandon "power, jurisdiction," from Latin bannum, "proclamation," which is from a Frankish or other Germanic word, from Proto-Germanic *bannan- "proclaim, summon, outlaw" (things all done by proclamation); see ban (v.).

Mettre sa forest à bandon was a feudal law phrase in the 13th cent. = mettre sa forêt à permission, i.e. to open it freely to any one for pasture or to cut wood in; hence the later sense of giving up one's rights for a time, letting go, leaving, abandoning. [Auguste Brachet, "An Etymological Dictionary of the French Language," transl. G.W. Kitchin, Oxford, 1878]

Meaning "to leave, desert, forsake (someone or something) in need" is from late 15c.  Related: Abandoned; abandoning.

Etymologically, the word carries a sense of "put (something) under someone else's control," and the earliest appearance of the word in English is as an adverb (mid-13c.) with the sense "under (one's) control," hence also "unrestricted."

Again, as that which is placed at the absolute command of one party must by the same act be entirely given up by the original possessor, it was an easy step from the sense of conferring the command of a thing upon some particular person to that of renouncing all claim to authority over the subject matter, without particular reference to the party into whose hands it might come ; and thus in modern times the word has come to be used almost exclusively in the sense renunciation or desertion. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]