Boche
a slang term born of the World War and used by the French, and later by others of the Allies, in referring contemptuously to their German enemies. Various explanations have been given of its origin, but the most plausible is that it comes from the French word caboche, used familiarly for head, especially a big thick head (slow-pate).
It had at times been used by Parisian printers before the war in speaking of their German assistants, because of their reputed slowness of comprehension. The next step was to apply it to Germans in general.