Nazi knob
n.— «Loarie’s Skatestopper, a device that keeps skateboarders off curbs and makes it impossible to slide down handrails, has been embraced by thousands of customers. …Skateboarders routinely conduct guerrilla missions to remove what they call “Nazi knobs” from their favorite skate spots.» —“Success is a mixed blessing for San Diegan whose invention has pushed boards off the curb” by Conor Dougherty Union-Tribune (San Diego, California) Dec. 5, 2003. (source: Double-Tongued Dictionary)