[CRConrad]

I'm almost certain it's the imperative form of a verb, in the grammar of the time apparently "to swien", "to be silent" or "to shut up".

"Swie!" from "swien" looks kind of exactly like "Schweige!" from "zu Schweigen" in modern German. Must go back to the same root (closer to "swie" than "schweigen", I'd guess) in Proto-Germanic.