to be derogatory. I looked it up just now...Historically was used to identify the typical blue collar people from the Pittsburgh region who often spoke with a heavy Pittsburghese accent. The term stems from the word YINZ, a second person plural pronoun brought to the area by early Scottish-Irish immigrants.
HEY I am both Scottish and Irish!! However my mother shamed me from using it. I don't say YINZ.