A clueless touron from Pennsylvania (nickname: the Keystone State), usually from some obscure small town in central PA, who vacations in Ocean City in the summer and drives like a complete and total jackass on Coastal Highway, blocking traffic by going 20 MPH or less, not paying attention to anyone else around them, and cutting across three lanes of traffic without warning to make a turn (a.k.a., a "Pennsylvania left-hand turn"). They are also infamous for asking idiotic questions like "Where's the beach?", "How do I get to the Boardwalk?", and "Have you ever heard of a place called Seacrets?".
"Dude, some keystoner just pulled a Pennsylvania left-hand turn on Coastal Highway and caused a three-car accident."
"Some keystoners just asked me how to get to the beach."
Can also be used as an adjective: "I would have been here a half hour ago, but I got keystoned on Coastal Highway. Damn tourons." (keystoned = surrounded by keystoners going 20 MPH, and unable to pass.)