Jerry was used by the British in
the Great War as a nickname for the Germans. One possible origin is that Jerry was thought to be a common name among Germans, like 'John' with Americans. Another, and perhaps the true origin, is that bed pans/chamber pots were called
Jerries, and the Germans helmets looked like them, so... Jerry was one of many nicknames used to call the Germans, the French preferred
Boche, the Americans,
Krauts. Another common one was 'Hun'. The usage of Jerry, and the rest, although I can't say I have seem much use of Boche outside of WW I related things, continued during the Second World War. The gas/water container used by the Germans in
WW II was nicknamed a Jerry can. During the Great War, the Germans, when talking to the Englishmen in the opposite trenches, would use Tommy to reference them, while the British, of course, used Jerry in regard to the Germans.
Example:
My high school had this great set of
WW II books, and I can remember this one story from a veteran that, for whatever reason, has
stuck with me. He and his unit had assaulted a German airfield. "We were running around securing the hangars. I passed the door to an office and
kicked it open. I tossed in a grenade and the Jerry behind the desk caught it and shouted, "Nein! Nein!" I yelled, "Ja! Ja!" and slammed the door shut."