A nickname for the German soldiers and aeroplanes. It was more commonly used amongst the English troops than Australians. Also used as a question. "Do you jerry" - do you understand: To "take a jerry" - change (for the better) ones [sic] course of conduct.
Jerry was a nickname given to World War II German soldiers, the German armed forces, or collectively the entirety of Nazi Germany. Although the nickname was originally created during World War I http://www.etymonline.com/index.php?term=Jerry, it didn't find common use until WWII. Other nicknames used for the German soldiers in WWI and WWII were Fritz, Hun, Kraut, and Boche http://www.firstworldwar.com/features/slang.htm.