Octopuses or Octopi?

So is it octopuses or octopi - and how do you know?

I'm often confused about how to pluralise words ending in -us, as there doesn't seem to be an easy rule. Then someone told me that words derived from greek go from -us to -uses and words derived from latin go from -us to -i! Simple! If only it were true, and if only words with latin derivations didn't then originally start from greek and vice versa.

Octopuses and Octopi

It's octopuses - because it's from the greek (you can tell it's greek because Octo means 8 in greek, aparently, and greek words go from "-us" to "-uses") wheras  cactus become cacti, because it's latin - having said that, biologists (and purists) would probably say octopodes.

Here are some others:

abacus -> abacuses, crocus -> crocuses, locus -> loci, fungus -> fungi

So now you know!

Comments

[Nick]

So what's the plural of Guinness then?
- As I always want more than one of thoses - hic!

[Nick]

P.S.
The Oxford English Dictionary lists octopuses, octopi, and octopodes in order of descending frequency of use.

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