I think it's a literal translation of "Beyond". The fact that this literal translation does not work at all is not something that has stopped GW before.
OK, maybe this requires a bit of longer explanation on my part
As a Polish, I can only analyse it from the polish perspective. Maybe it makes more sense in russian or bulgarian.
Warhammer wiki states that Za is not a word for Chaos itself, but rather for the Chaos Realm, Aethyr. And it's supposed to mean "From beyond". Now, while "za" is a correct (in some uses) translation of "beyond", it is not a correct translation of "from beyond". It would be "spoza" or "zza" (I think in russian it's "из-за" - "iz-za") . Maybe GW chose "za" as a common part of polish and russian translation? But there's more...
English is a very flexible language, you can verb almost any noun (see how I just used "verb" even tough it's a noun?
) and noun verbs, adjectives and so on.. so while in english, calling the Chaos Realm "From Beyond", "Beyond", or something like that works, in polish it just does not. We do sometimes turn adjectives or adpositions into nouns, but very rarely it's done directly. Moreover, now that bit is my personal feeling, we have very few such short nouns and so they feel weird and unnatural. Like it's a foreign name. So personally, I feel it should almost never be called simply "Za", but rather "Ląd Poza", "Kraina Zza" or maybe "Zakraina" (I actualy like that one).
Now, one could argue, that ages ago, it was indeed called "Kraina Zza", which then due to language drift was shortened to "Za" and then extended to all of Chaos. But I don't buy that.