mid-14c., auctor, autour, autor "father, creator, one who brings about, one who makes or creates" someone or something, from Old French auctor, autor "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," etymologically "one who causes to grow." It is an agent noun from auctus, past participle of augere "to increase" (from PIE root *aug- (1) "to increase").
It is attested in English 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," which is now archaic but has the sense that is in authority, etc. In Middle English the word sometimes was confused with actor.
Assimilation of Latin -ct- to -t- began by 1c. in parts of Italy, and autor is attested in late classical Latin, though considered erroneous. The Appendix Probi, a list of correct spellings from perhaps 3c., has auctor non autor.
The changed to -th- began in English by early 15c. and mostly was accomplished 16c. It was done on the model of Medieval Latin which mistakenly assumed a Greek origin for the word and confused it with the unrelated source of authentic. Also see th.
[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]
