I'm not too sure about that. It's also used as an expression of mock shock or horror, something like: Nnnnnnnnnnnnnyyyyyyyyyyyorrrrrrrrrrrrrrrr. Also, people who watch Anime use it. Normal people probably don't, although might pick it up from people who watch Anime. If, for example, you saw a "panty-shot" of a Nekobus, it would probably be an appropriate exclaimation. --AR
I'm pretty sure it's intended to be a cat sound - meaning is conveyed entirely by tone. Contrast Happiness-nyo! nyo? And the aforementioned. SeeAlsoDigiCharat. SeeAlso Psychiatrist ;)
Google seems to confirm that it is a miaow sound, although I've never heard a real cat make a noise that sounds anything like "nyo"! Erm, also it is a useful exclamation if you want to say: "The stream of urine has gotten small" or "I have trouble making water" in Japanese: "Nyo no de ga hosoku narimashita." or "Nyo ga denikui.", although I should imagine that "Nyo" alone ought to be enough. --AR
You've probably never heard them go 'Ron ron ron' either. I guess that each language transliterates animal sounds very differently. --Vitenka
Indeed. That probably negates some linguistic theories of onomatopaeias, but I don't know enough about that to comment further. --AR
Well, rememebr that each language has its own sounds to give to the latin alphabet. 'Purr' probably isn't something you can pronounce in french without it sounding more like 'pear'. --Vitenka
Japanese prostate problems: just what you want to be made to think about on a Monday morning.