You shouldn’t assume I don’t know what I’m talking about just because I didn’t take the time to include references in my comment. I have a graduate degree in linguistics and one of my concentrations is word formation. While it’s true that radii is definitely a plural form of radius, it’s not really the generally accepted plural anymore. (At least, not where I live in America, and at least in spoken language. I’m not sure about Britain.) Neither are indices, data, corpora, etc. Highly technical people like hackers and engineers continue to use the fossilized plurals, because these forms are part of the prestige dialect that indicates to other people that you have been educated and can remember to use non-standard plurals. But normal people do not use these forms—they simply use the standard English pluralization rules because there is no reason not to. The poster who mentioned the relative density of each term in Google’s indexes (sorry, couldn’t resist) would have to somehow normalize the numbers to account for the fact that technical people are disproportionately more likely to be using these nouns in the first place, and so a simple tally doesn’t show the real picture if we’re talking about how the language is used ‘in general’.

I don’t have the book with me so I can’t find the exact reference, but if you’re truly interested there is an entire chapter in ‘An Introduction to Modern English Word Formation’ by Valerie Adams that talks about how these plural forms arose in English. They were basically made up by scientists and philosophers during the Middle Ages who felt that any English word with a Latinate or Greek root should use the corresponding root language’s plural form, because Latin and Greek were more ‘pure’ languages. (No doubt this attitude contributed to the fact that they continued writing in Latin in general for a thousand years after Rome was no more.) These forms became fossilized in English because the people using them were educated and therefore part of what was considered a prestigious class. That is, using them makes you sound smart, and that’s why they’ve persisted. By my own judgment, though, they certainly sound more out of place to normal English speakers today than the corresponding standard English plural. (I last read this book six years ago, so this summary is from memory—the details may be slightly different but I’m pretty certain that is the general gist of it.)

The reason this irks me in general is because we speak English, not Latin. Unless you speak Latin you probably don’t know how to correctly form Latin plurals, which follow a relatively complicated scheme depending on the class of noun, its grammatical case, person, and number. See http://itre.cis.upenn.edu/~myl/languagelog/archives/000814.h... for a discussion. Japanese doesn’t have a plural form—does that mean we should say in English things like, ‘Hiroki is holding three kimono in his arms’? The only reason it sounds remotely acceptable is because you interpret kimono as a mass noun that doesn’t have a plural form (like rice), but physically speaking kimonos aren’t like rice in that they are actually countable. The sentence ‘Hiroki is holding three kimonos in his arms’ sounds much better.

Modern English has a very simple pluralization rule. There are three allophones of the plural morpheme: the unvoiced allophone –s is suffixed to words ending with an unvoiced phoneme (t, p, k, f, etc.); the voiced allophone –z is added to words ending with a voiced phoneme (d, b, g, m, all vowels, etc.); and the allophone –əz is added to words ending with a phoneme that is similar in articulation features to s and z (s, z, sh, ch, j, etc.). There are exceptions to this rule that show extreme resistance to degradation (such as children and teeth), but in general the exceptions are slowly being replaced by the standard plural in normal speech. If someone said ‘Look at all those oxes over there,’ most people wouldn’t blink an eye.

Finally, I wasn’t trying to correct the OP. My point was simply that the jokes where you make up a funny non-standard plural are lame. As mentioned in the excerpt from the Jargon File that someone posted above, hackers (and railroad enthusiasts, among others) have been making these same jokes for decades. A variation over and over again for fifty years of the same old joke about a funny fake plural—enough already, ya know? Of course I’m aware that everyone’s sense of humor is different, so I can’t tell someone something isn’t funny because it’s a subjective assessment. So I’m just pointing out that these jokes aren’t funny to me, because the people making them might not realize how overplayed and unfunny they are to a lot of people.

And please don’t get me started on made-up fanciful names for groups of animals…

EDIT: See also http://en.wikipedia.org/wiki/English_plural#Irregular_plural... for more discussion.