The word down, as such a ubiquitous word (being a preposition, adverb, adjective, verb, and noun), initially doesn't seem like it would have much of an interesting history, but it's actually really cool. There are records going back to the earliest days of Old English, attesting it with a bunch of different spellings, including doune, duna, downe, and dun. In some of its earliest forms, it was actually spelled adun, but the first unstressed vowel was lossed, possibly because of confusion with the phrase a dun and the word adun. Adun traces to ofdune, which literally translates to "off the hill". The main part there is dune (also from whence we got English dune), meaning "hill", and that derives from a Proto-Indo-European word sounding like dheue and meaning "closed". Of is the etymon of English off and of and eventually comes from PIE apo, meaning "away". This etymology in particular kind of makes a lot of sense.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.