LORELAI: I warned him. I warned him when I first met him, if he hurt her … Ah. Maybe I could key his car.
LUKE: Or better yet, you can key Taylor’s car, and tell him Dean did it.
To key a car means to use the sharp point of a key to scrap across someone’s car windshield or paintwork in order to damage it; often done out of spite as an act of petty vandalism.