What is Keystoner?
1.
A clueless
"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.)
See