A colleague just quoted from a website on which genie is pluralized as genii.
No, it’s not correct. Genie does derive ultimately from Latin genius (which can be pluralized as genii or as geniuses), but it came to us by way of French, and it’s an English word now, so it’s genies.
But this reminds me of a much larger issue that needs to be addressed: the idea – a rather common one, it seems to me – that there is a Latin plural ending -ii that should be applied to Latin-seeming (and some other foreign-seeming) words. I see it, for instance, when some people write virii instead of viruses.
To be as plain as possible: in Latin, -ii is not a plural ending. Ever. Nor is it one in English (unless this pseudoplural catches on, I guess…). In fact, I can’t think of a language in which it is a plural ending, though there might be one somewhere. Not English, though!
No doubt some of you are saying, “Hey! That’s wrong! What about genii for genius and radii for radius?” Well, what’s the root? Look at the singular: genius. That’s geni + us. Pluralize -us to -i and you get genii. Likewise radi-us to radi-i and so on. The pluralization, in Latin, of -us masculine nouns is -i. If the root ends in i then you get a double i, but that’s because of the root. The Latin plural of virus would be viri; the singular is not virius.
But also, if it’s not a Latin word ending in -us, don’t assume it pluralizes to -i. (It may, as -i plural marking is also used in some other languages, but don’t assume it.) Moreover, if it’s an English word, especially one originally borrowed some time ago from another language, you’re good with treating it as an English word – for example, viruses. You wouldn’t use Latin genitive formations instead of English possessives; you don’t conjugate verbs borrowed from Latin in the Latin way. Unless there’s a well-established borrowed plural, you’re just fine using the good old English -(e)s.
I’m put in mind of Larry Niven’s novels, in which there is a type of creature called bandersnatch. Most of you will recognize this as a word invented by Lewis Carroll for his poem “Jabberwocky” in Through the Looking-Glass. The word is an English invention. The characters in Niven’s books were given the name by an English-speaking biologist, and the characters all speak English. So what’s the plural of bandersnatch in Niven’s world? Not bandersnatches but the more exotic- or technical-sounding bandersnatchi. Apparently -(e)s is too pedestrian and low-level (I really think many people think it is!).
I would venture to suggest that if Niven were creating the creature just now, he might go one step farther and pluralize it bandersnatchii. Which would, of course, be even more frumious.