The "What's this word/phrase?" thread

I just have a really simple question about a word I can't find in the dictionary. Since it feels like a bit of a waste to make a new thread just for this, maybe it would be a good idea for this to be an ongoing thread where anyone can get help for the words and phrases that they are having a problem with.

So I'll start it off.
From the manga よつばと, a character used the word 「うす」. I can tell from the context that its a greeting, and probably pretty casual. Where does this word come from? Is it some sort of recent slang, or some particular dialect? Maybe a contraction of something?

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