octopus (n.)
1758, genus name of a type of eight-armed cephalopod mollusks, from Greek oktopous, literally "eight-footed," from okto "eight" (see eight) + pous "foot" (see foot (n.)). Proper plural is octopodes, though octopuses probably works better in English. Octopi is from mistaken assumption that -us in this word is the Latin noun ending that takes -i in plural.