The term Boche refers to the “universal French name during the First World war for the Germans, taken up in England by the public in general and the Press. The nicknames Jerry and Fritz were more usual terms with the Army and Navy, and Hun with the Air Force. The word Boche first appeared about 1860, as low-class Parisian slang mauvais sujet, meaning "bad subject," and it was not until some time after the Franco-Prussian War on 1870-71, during which the enemy were always spoken of as "les Prussiens," that the word Boche came to be generally applied in France to a German. ” 
Boche is a word of unknown origin. It is possible it can be traced to “French Allemand "German," in eastern French Al(le)moche, altered contemptuously to Alboche by association with caboche, a slang word for "head," literally "cabbage" (compare tete de boche, French for "German" in an 1887 slang dictionary). All the French terms are no older than mid-19c.” 
References / notes
Glossary of terms and customs
This page forms part of our glossary of words and phrases of the Armed Forces of the United Kingdom of Great Britain during the Great War, which also includes: technicalities, trench slang, expressions in everyday use, nicknames, sobriquets, the titles and origins of British and Commonwealth Regiments, and warfare in general. Please feel free to help expand and improve this content.
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