Piss bag is a commonly used term for a 5 gallon black rubber backpack water pump used in wildland firefighting. Other terms are bladder bagand Fedco(the name of one maker of the bag). The bag has adjustable straps, padded on the better versions, and is carried on the back like a backpack. It has an opening at the top for filling, and a small hand pump attached to the bottom of the bag. When filled with water, it weighs 40 pounds and the hand pump can shoot water 20 feet or so, depending on the nozzle used.
Piss bags are used on fires where a plentiful water supply is not available and water must be conserved. A very effective use of the bladder bag is when firefighters work in two person teams, one carrying and squirting the water on hot spots and still burning fuels, and the other using a shovel or pulaski to stir the embers and mix them with dirt. When all the water in the bag is used, the firefighter may refill it from a water source, or get a new, filled bag. In recent years, full bags are often lowered to firefighters in slings by helicopters.