I was told by a Chinese colleague that this is a very informal greeting, strictly between friends - pals, rather.

I didn't ask, at the time, because I was more preoccupied with trying to pronounce it (my colleague was very helpful). But I would like to know the context behind this. I mean, what is it in asking about having eaten that signifies a greeting?

In Greek, for instance, we say "yia sou" ("γειά σου"), for "hello". It means, basically, "have health". To me (well of course it would) it makes sense to wish something good as a greeting, I think "shalom" for example, means "peace", etc. I don't get the "have you eaten" greeting and I'm very very curious. All help in this matter is deeply appreciated :)