A malpropism for jury-rigged, it implies a shoddy repair job, based on the idea that Germans (in the minds of American GIs) couldn't do very good field maintenance. Since it predates WWII, it has little to do with the pejorative "Jerry" for Germans, no matter what your grandfather tells you. It is possible that this mis-spoken version came about as the result of the slang, but jury-rigged was around first.
MacGyver is the patron saint of jury-riggers. Duct tape, gaffer tape, bubble gum, household chemicals and WD-40 are all big elements in this meatspace version of hacking.