mid-14c., auctor, autour, autor "father, creator, one who brings about, one who makes or creates" someone or something, from Old French auctor, acteor "author, originator, creator, instigator" (12c., Modern French auteur) and directly from Latin auctor "promoter, producer, father, progenitor; builder, founder; trustworthy writer, authority; historian; performer, doer; responsible person, teacher," literally "one who causes to grow," agent noun from auctus, past participle of augere "to increase," from PIE root *aug- (1) "to increase."
From late 14c. as "a writer, one who sets forth written statements, original composer of a writing" (as distinguished from a compiler, translator, copyist, etc.). Also from late 14c. as "source of authoritative information or opinion," now archaic but the sense behind authority, etc. In Middle English the word was sometimes confused with actor. The -t- changed to -th- 16c., on model of change in Medieval Latin, on mistaken assumption of Greek origin and confusion with authentic.
...[W]riting means revealing oneself to excess .... This is why one can never be alone enough when one writes, why even night is not night enough. ... I have often thought that the best mode of life for me would be to sit in the innermost room of a spacious locked cellar with my writing things and a lamp. Food would be brought and always put down far away from my room, outside the cellar's outermost door. The walk to my food, in my dressing gown, through the vaulted cellars, would be my only exercise. I would then return to my table, eat slowly and with deliberation, then start writing again at once. And how I would write! From what depths I would drag it up! [Franz Kafka, "Letters to Felice," 1913]
late 14c., auctorisen, autorisen, "give formal approval or sanction to," also "confirm as authentic or true; regard (a book) as correct or trustworthy," from Old French autoriser, auctoriser "authorize, give authority to" (12c.) and directly from Medieval Latin auctorizare, from auctor (see author (n.)). Meaning "give authority or legal power to" is from mid-15c. Modern spelling from late 16c. Related: Authorized; authorizing. Authorized Version as a popular name for the 1611 ("King James") English Bible is by 1811.
From c. 1300 in the general sense "legal validity," also "authoritative book; authoritative doctrine" (opposed to reason or experience); "author whose statements are regarded as correct." From mid-14c. as "right to rule or command, power to enforce obedience, power or right to command or act." In Middle English also "power derived from good reputation; power to convince people, capacity for inspiring trust." From c. 1400 as "official sanction, authorization." Meaning "persons in authority" is from 1610s; Authorities "those in charge, those with police powers" is recorded from mid-19c.
A sound found chiefly in words of Old English, Old Norse or Greek origin, unpronounceable by Normans and many other Europeans. In Greek, the sound corresponds etymologically to Sanskrit -dh- and English -d-; and it was represented graphically by -TH- and at first pronounced as a true aspirate (as still in English outhouse, shithead, etc.).
But by 2c. B.C.E. the Greek letter theta was in universal use and had the modern "-th-" sound. Latin had neither the letter nor the sound, however, and the Romans represented Greek theta by -TH-, which they generally pronounced, at least in Late Latin, as simple "-t-" (passed down to Romanic languages, as in Spanish termal "thermal," teoria "theory," teatro "theater").
In Germanic languages it represents PIE *-t- and was common at the start of words or after stressed vowels. To represent it, Old English and Old Norse used the characters ð "eth" (a modified form of -d-) and þ "thorn," which originally was a rune. Old English, unlike Old Norse, seems never to have standardized which of the two versions of the sound ("hard" and "soft") was represented by which of the two letters.
The digraph -th- sometimes appears in early Old English, on the Roman model, and it returned in Middle English with the French scribes, driving out eth by c. 1250, but thorn persisted, especially in demonstratives (þat, þe, þis, etc.), even as other words were being spelled with -th-. The advent of printing dealt its death-blow, however, as types were imported from continental founders, who had no thorn. For a time y was used in its place (especially in Scotland), because it had a similar shape, hence ye for the in historical tourist trap Ye Olde _______ Shoppe (it never was pronounced "ye," only spelled that way).
The awareness that some Latin words in t- were from Greek th- encouraged over-correction in English and created unetymological forms such as Thames and author, while some words borrowed from Romanic languages preserve, on the Roman model, the Greek -th- spelling but the simple Latin "t" pronunciation (as in Thomas and thyme).