Octopus – Of course, the Octo is easy, it’s the same oct as used in octagon and means eight. But what about the ‘pus’? Actually the ‘pus’ like the octo is Greek and means ‘legged’, it has the same root as ped (as in biped).
The root by the way is pous, which dates back to Proto-Indo-European (PIE) – the common ancestor of all Indian and European languages.
And the plural? Octopuses. Both octopuses and octopi are acceptable of course as both are in common usage, but octopi was born out of the mistaken belief that the word has the common Latin word ending ‘us’ where in fact the pus is part of the word itself it’s similarity is just a coincidence.