1758, genus name of a type of eight-armed cephalopod mollusks, from Latinized form of Greek oktōpous, literally "eight-foot," from oktō "eight" (see eight) + pous "foot," from PIE root *ped- "foot."
The more usual Greek word seems to have been polypous (also pōlyps), from polys "many" + pous, but for this word Thompson ["Glossary of Greek Fishes," 1947] suggests folk-etymology and a non-Hellenic origin.
The classically correct Greek plural (had the word been used in this sense in ancient Greek) would be octopodes. Octopi (1817) regards the -us in this word as the Latin noun ending that takes -i in plural. Like many modern scientific names of creatures, it was formed in Modern Latin from Greek elements, so it might be allowed to partake of Latin grammar in forming the plural. But it probably is best to let such words follow the grammar of the language that uses them, and octopuses probably works best in English (unless one wishes also to sanction diplodoci for the dinosaurs).
Used figuratively since at least 1882 of powers having far-reaching influence (usually as considered harmful and destructive). To the ancients, the octopus was crafty and dangerous, thrifty (stores food in its nest), and proverbial of clever and adaptable men, based on the animal's instinct of changing color when frightened or for disguise.
It also was thought to be amphibious and to climb trees near shores to steal grapes and olives (the giant ones were said to raid whole warehouses). Thompson writes that "the eggs look remarkably like ripe olives; hence the story."