"Dumbth" is a word coined by the late humorist Steve Allen.
In a northern New Jersey town recently, police responded around 1:00 AM to a residence on a request for medical assistance for a man who had been injured in a fall. A subsequent investigation revealed that the involved party had in fact been injured in a motor vehicle crash that had occurred shortly before the call. This resulted in the arrest of the 22 year-old subject on charges of driving while intoxicated, careless driving, leaving the scene and failing to report a motor vehicle accident.
When you do a stupid thing – driving while drunk – there is no way you are going to "get away with it" when you crash your car. To attempt to do so is to exhibit dumbth.