Spanish and Crows.

But surely it refers to an understood/implied feminine plural noun?

In Mexico I've heard "buenas" as a reply to or substitute for "buenas tardes" or "buenas noches" but not for "buenos días," which is generally said in full. Elderly and rural people sometimes say "buen día le dé Dios", which sounds really formal and old fashioned.

#10, yes, I think so ... perhaps to "buenas cosas"

I (Portugal) Portuguese, in a very informal way, you can say the same: "Boas"

Thanks caiuss.

Nobody would say "buenas cosas" as a greeting, so it's unlikely they would use an abbreviated version of that phrase.

I reckon it´s probably related to the word ´días´, and it is now popularised as a greeting mode. Still it´s very colloquial and never replaces ´buenas noches´.

You say "buenas" when you'd say "hello", even if it's night ... so it can replace "buenas noches".
But "buenas noches" has a slightly different meaning from "good night". I think you only say "good night" when you mean good bye (am I right?), while in Spanish, if you meet somebody at 11 pm, you say "buenas noches".

You may say "hola, buenas" but never "adios, buenas", wherever the time is.

"Buenas noches" can be either "good evening" (greeting) or "good night" (departure). I agree that you wouldn't say "buenas" when someone is leaving.

Yes, in English 'good night' is used when you're taking leave of someone - very late in the day or just before going to bed - but not as a way of saying hello.

In Joyce's story The Dead, set in Dublin sometime around 1910 (I would guess), "Good night" is used as a greeting for people arriving. I don't know how common it was in Ireland then. I've never seen it anywhere else including in Joyce.