Why are Germans called Boche? It’s from the French caboche which means big, thick head. The Paris underworld started using it in 1860s to mean disagreeable person. In the late 19th century Parisian printers started using it to refer to their German assistants who had trouble understanding their directions and reacted slowly.
Then, in the 20th century, Germany started to attack other nations at random, which was a rather thick-headed thing to do.