this sweatshirt (courtesy of Adrian) should clear up any uncertainty.
That is, if you know that the plural form of “you” colloquially used in Pittsburgh is “yinz” (or “yunz” – a contraction of “you ones”). Thus, a person who says this word is a “yinzer”. I get the sense that some people may view this word as an insult, to describe someone who’s not well-educated and talks like a moron. However, I know that it’s worn as a badge of pride by many Pittsburgh expatriates who are proud of where they come from, since one of the most distinguishing and recongnizable aspects of Pittsburgh is its dialect. What many people refer to as “Pittsburghese” is part of a larger dialect region known to linguists as North Midland U.S. English. If you’re interested in a reasonably scholarly examination of speech in Pittsburgh, see this webpage, put together by a professor at Carnegie Mellon University. If you’re interested in finding words that are characteristic of colloquial speech in Pittsburgh, google “Pittsburghese” – there are myriad websites cataloging the various particularities of Pittsburgh speech, at least some of which come from the Scots-Irish, who settled the region after the Revolutionary War.
[Insert Witty Title Here] 
Yin is an actual Scottish word, meaning one. So I generally spell yins with an s unless I’m going for the humor card. Great sweatshirt. [insert sentences in Pittsburghese here]
You’re correct. To be honest, I hadn’t researched the etymology of “yinz”. However, my mom would contend that it most likely came about as contraction of “you ones.” Here is why my mom’s opinion is relevant: My parents grew up in Northern Ireland and she is from an area where a dialect of Scots known as Ulster Scots is spoken. A common word in this area is “wean” (pronounced like way-un). This is a colloquial word for a child and comes about as a contraction for “wee one”. So, it seems plausible that “yinz” could come from “you ones” given this similar example.
Finally, I googled “you ones” to see if that might shed some light on the issue. Interestingly, this led me to the wikipedia page for, you guessed it, “yinz”. The page suggests the following etymology for “yinz”: you ones > you’uns > youns > yunz > yinz. So, there it is, for what it’s worth.
From my experience going back to Ireland and visiting relatives, I grew to appreciate Ulster Scots and its colorful expressions. If you’re interested, there’s a great overview of Ulster Scots along with a vocabulary guide for Ulster Scots complete with sound clips of people saying the phrases. If you’ve never heard this kind of thing before, you’ll have no idea what they’re talking about! That said, I just love listening to some of these clips because it makes me feel like I’m sitting in my aunt’s living room listening to her family talk.