Someone who deliberately, actively and habitually tries to look up women's and girls skirts to see their panties. Note that this is different from the casual voyeur who just enjoys the occassional accidental flash of a girls knickers. upskirt voyeur flashing
I overheard one girl say to her friend. "Don't ever walk upstairs or ride the escalator in front of John ****, he's a real "Panty Peeper"