{i} locking device attached to the wheel of a car to immobilize it (used especially for cars parked illegally or until the owner pays parking fines that accumulated)
A Denver boot is a large metal device which is fitted to the wheel of an illegally parked car or other vehicle in order to prevent it from being driven away. The driver has to pay to have the device removed. A device locked to the wheel of a parked vehicle to keep it from being driven, used especially to force settlement of outstanding traffic violations