This is another piece of mine on military unit names. It kind of follows along with the last one I did.
For the etymology, I thought back to the overall theme, of a [villainous] group with a kind of bizarre obsession with the ancient and traditional. The name of an army/army group equivalent would be from a (likely ancient) language, the word taken directly as a loan rather than adapted. And it would not be any directly military-related one there, but something like “assembly” or “federation”.
The image invoked is images of ancient peoples, on the steppes or in the forests, the kind unfairly referred to as “barbarians” by outsiders, forming a coalition of their warriors to campaign. And the reason I got there, or how the name-developers got there in-universe, has to do with the nature of such ultra-large units.
Regardless of national culture or doctrine, extremely large units like army groups and/or field armies are always considerably more ad hoc than the smaller ones in the same country. They’re determined by resources and location. So if it’s ad-hoc, a little like an ancient coalition, that opens the door for it to receive the name of one.
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Of course, the question is how much an author should use a possibly confusing author-coined name rather than a familiar one. It’s tough to answer, but is easier in that the names this term would be replacing are themselves looser and to many people more unfamiliar than clearer small units.