Probably the coolest part of any fire engine. The large nozzle mounted on the top (usually behind the cab) of the apparatus that can shoot water ~150-200 feet in the air with ease. Also good for knocking down walls and other implements of destruction. Many can flow over 2500 gallons of water per minute. Also referred to as a "monitor".
Fireman Jake used the deck gun to wash down the cop shop down the street.
We used the deck gun to throw water into the tenth floor of a building.
They accidently put a hole in the wall of the training center with the deck gun.