Boche n.

"German soldier in World War I," 1914, perhaps from French slang boche "rascal," applied to the Germans; a word of unknown origin. Another theory traces it 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). None of the French terms is older than mid-19c.