Why is the ninth month called September?
late 14c., "to give up, surrender (oneself or something), give over utterly; to yield (oneself) utterly (to religion, fornication, etc.)," from Old French abandoner (12c.), from adverbial phrase à bandon "at will, at discretion," from à "at, to" (see ad-) + bandon "power, jurisdiction," from Latin bannum, "proclamation," from a Frankish word related to 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]Etymologically, the word carries a sense of "put someone under someone else's control." Meaning "to give up absolutely" is from late 14c. Related: Abandoned; abandoning.
"a letting loose, surrender to natural impulses," 1822, from a sense in French abandon (see abandon (v.). Borrowed earlier (c.1400) from French in a sense "(someone's) control;" and cf. Middle English adverbial phrase at abandon, i.e. "recklessly," attested from late 14c.