Question: 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.
Similar to English, you can abbreviate common words with slang. I've heard おはよう become おっす, and similar. There isn't even a す in おはよう.