late 14c., autumpne (modern form from 16c.), from Old French autumpne, automne (13c.), from Latin autumnus (also auctumnus, perhaps influenced by auctus "increase"), of unknown origin. Perhaps from Etruscan, but Tucker suggests a meaning "drying-up season" and a root in *auq- (which would suggest the form in -c- was the original) and compares archaic English sere-month "August."
Harvest was the English name for the season until autumn began to displace it 16c. In Britain, the season is popularly August through October; in U.S., September through November. Cf. Italian autunno, Spanish otoño, Portuguese outono, all from the Latin word. Unlike the other three seasons, its names across the Indo-European languages leave no evidence that there ever was a common word for it.
Many "autumn" words mean "end, end of summer," or "harvest." Cf. also Lithuanian ruduo "autumn," from rudas "reddish," in reference to leaves; Old Irish fogamar, literally "under-winter."