A Johnypump is a colloquial term primarily used in New York City to refer to a fire hydrant that has been illegally opened to provide water for various purposes, such as cooling off during hot weather or filling kiddie pools. The term originated from a combination of "Johnny," referring to a common name in the area, and "pump," relating to the water source. While it can offer relief during summer, using a Johnypump is illegal and poses risks, such as reduced water pressure for firefighting.