People not born in or near Boston often get confused by a native's accent and expressions or as I like to refer to it the "Boston SLANGuage".
If you're not from the area you may even struggle to say this expression correctly. Don't be shy, its impossible to over-emphasize the second syllable of "Pissa". Incidentally, it is also correct to write this as "Pissah" in my book.
If you hear someone say "Wicked Pissa" what they are saying is something is really awesome. For example, watching the Celtics make the Lakers cry uncle in Sunday afternoon's game this past week was a wicked pissa.