autumn:  English acquired autumn from Latin autumnus, partly via Old French autompne. Where Latin got the word from is a mystery; it may have been a borrowing from Etruscan, a long-extinct pre-Roman language of the Italian peninsula. In Old English, the term for ‘autumn’ was harvest, and this remained in common use throughout the Middle Ages; it was not until the 16th century that autumn really began to replace it (at the same time as harvest began to be applied more commonly to the gathering of crops). Fall, now the main US term for ‘autumn’, is 16th-century too.
late 14c., autumpne (modern form from 16c.), from Old French autumpne, automne (13c.), from Latin autumnus (also auctumnus, perhaps influenced by auctus "increase"), which is 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. Compare 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." Compare also Lithuanian ruduo "autumn," from rudas "reddish," in reference to leaves; Old Irish fogamar, literally "under-winter."