A slovenly fop; one finely dressed, but dirty. This term's also used to refer to mobsters, who appear nice on the surface, but are nasty thugs underneath.
Sure, the capo's son dresses nice, but he's a beau-nasty brute underneath that suit...
To punk nasty someone is to take their backpack, remove the contents, turn the backpack inside-out, replace the contents, and then zip it back up. To double punk nasty a pair of people, one punk nastys each person, but swaps the contents of their packs.