"By this time Jerry had got onto us with his mortars, and was giving us a hell of a time."
-Diary of Lance-Corporal John James Bird, 1943.
A nickname for German soldiers and aeroplanes used mostly by British soldiers during the First and Second World War. Probably derived from German(y), but a more interesting theory suggests the term descended from the British belief that the wide-brimmed German helmets resembled chamber pots, which were known in slang as jerries.
On a similar note, Jerry up! was a First World War cry of warning that a German bomber plane about to drop its load was flying overhead.
sources:
www.bartleby.com/61/29/J0032900.html
www.anu.edu.au/andc/res/aehist/wwi/J.php
www.peevish.co.uk/slang/j.htm