It's just a made-up term for the unit. I believe there are a few experts of Celtic languages in the research department, who have simply combined "man" and "daemons" (uiro and dusios?). As far as a reference to these naked non-elite warriors, shouldn't be too hard to find, although I don't know about a reference in classical literature. I imagine Gaesatae inspired lots of these wannabees, though they probably didn't form a cohesive unit.
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