copula - What does っす at the end of a sentence mean?

I saw a sentence like:

「それ使{つか}ってるっす。」

and none of my dictionaries have an entry for just "っす". Is it a verb form, gobi, or something else?

Answer

It's a contraction of です. It's not quite as polite as that though - it's always sounded a bit like "thinking that one needs to be polite but not bothering to do it properly" to me. I guess it comes somewhere between teineigo-level polite and casual in the politeness spectrum.