A requested word, octopus seems pretty self-explanatory. In the eighteenth century, it was coined by Carolus Linnaeus as the genus name for that type of mollusk. This comes directly from the Greek word oktopous (surprising but not uncommon for Linnaeus, who favored Latin), which was itself a portmanteau of okto ("eight") and pous ("feet"). Okto is from Proto-Hellenic okto, from Proto-Indo-European oktow, which might itself stem from the earlier PIE word for "four" (since they counted on one hand, I suppose?), hokto. Meanwhile, pous was also developing from Proto-Indo-European, in this case from the root ped, or "to walk". Though this is outside the diachronic realm, I'd like to make a quick note about the pluralization of octopus: octopi is incorrect; it adds a Latin suffix to the Greek word. Though it's not really important, if you want accuracy, go for octopuses or octopodes.
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.