A cupcake party is similar to going, egging, where a couple of people get in a car, and throw cupcakes at pedestrians. if it all goes well the frosting on top of the cupcake will stick to the victim.
colored frosting add to the fun, as they will stain the victim with a nice big colored spot.
originating in Bamberg, Germany
"cupcake party dude?" "uhm ya ... im in. your car or mine?"