We were watching Makoto’s Wiggles DVD (highly recommended BTW) and when mentioning Henry the Octopus’ friends they repeatedly said Octopi as the plural of Octopus. I hadn’t heard that before and since I happened to be reading from the Fowler’s Modern English Usage I decided to turn to the O section, then OC, then OCT, then - well, you get it.

Here is what it says:

Octopus. The only acceptable pl. in English is octopuses. The Greek original is ὀκτώπους, -ποδ- (which would lead to a pedantic English pl. form octopodes). The pl. form octopi, which is occasionally heard (mostly in jocular use), though based on modL (Modern Latin) octopus, is misconceived.

As a clincher my firefox auto spellchecker has a red line under ‘octopi’ so it must be wrong!

Despite all this evidence, octopi seems to enjoy some breadth of use. A Google search of ‘octopuses’ yields about 422,000 hits; AND ‘octopi’ gets 408,000 hits (admittedly, a bunch of those are saying it is incorrect).

This may be an example of using something that sounds correct but is actually wrong. For example, using ‘whom’ when you should use ‘who’. Of course this opens a can of octopi about what is wrong or right in language - or if there is even such a thing.

By the way, the plural of タコ in Japanese is タコ.

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See Ask Oxford and Wikipedia’s entry under ‘terminology’