Whats a Jerry?

Playing Call of Duty 2, the British troops call the German Troops “Jerries”. Searching around on the net didn’t yield much, but I asked my Dad, and he asked a WWII vet, and here is what I got:

The term Jerry stems from WWI, the Germans used their helments to goto the bathroom in, just like we call the bathroom the “John”

Not sure if it is true, but its a good explination.