So what does it stand for?

[Nitro Vordex]

If there's an official name for this, let me know.

If not, what do you think MAG could stand for? What I basically thought was Machine Augmented Guardian, or something else for the G.

[Fox2Tails]

I don't think MAGs ever had a name, it was one of those words that was in capitals like PSU's TECHNIC

[Dhylec]

MAG, Mag or mag doesn't really stand for anything. It's a katakana "magu" in JP (マグ) , but that didn't stop many people coming up w/various names for this MAG acronym. Some were interesting, other were humorous.
I remember a couple: My Awesome Granny, My Ace Guardian. ;3

Search either PSO Gen or FKL for the old MAG threads.

[Vanzazikon]

My Awesome GrannyAww yeah, that's exactly what my Rati is.