octopus (n.)

1758, genus name of a type of eight-armed cephalopod mollusks, from Greek oktopous, literally "eight-foot," from okto "eight" (see eight) + pous "foot," from PIE root *ped- "foot." Proper plural is octopodes, though octopuses probably works better in English. Octopi (1817) is from mistaken assumption that -us in this word is the Latin noun ending that takes -i in plural.

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