I have a friend who says pluralising "octopus" as "octopi" shows ignorance of three languages - English, Latin, and Greek. "Octopus" is of Greek origin and the Greek plural is "octopodes". The Latin plural is "polyporum" (the singular being "polypus"). The English plural is "octopuses" as we add "s" or "es" when pluralising.
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I have a friend who says pluralising "octopus" as "octopi" shows ignorance of three languages - English, Latin, and Greek. "Octopus" is of Greek origin and the Greek plural is "octopodes". The Latin plural is "polyporum" (the singular being "polypus"). The English plural is "octopuses" as we add "s" or "es" when pluralising.
'polyporum' would be genitive plural 'of the octopuses'. The more usual nominative plural would 'polypi'
On the other hand, you could say that not accepting "octopi" as a plural of "octopus" shows 'ignorance' of languages in general.