Don't think this is even remotely true. Native Philadelphians, of all races, have been using jawn pretty heavily since at least the early/mid 90s. I'd be interested in which working class neighborhoods you lived in, nolibs/fishtown kind of suggests you may have been surrounded by a lot of other transplants. edit: There is a generational aspect to it as my parents and grandparents never said it. 80s/90s babies for sure. Maybe the people you encountered were relatively young (or old)