An Etymological Dictionary of the German Language/Annotated/Herzog
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Herzog, masculine, ‘duke,’ from the equivalent Middle High German hęrzoge, Old High German hęrizogo (-zoho), masculine; compare Old Saxon hęritogo, Anglo-Saxon hęretoga, masculine, Old Icelandic hertoge, masculine; a common Teutonic term for ‘the leader of an army,’ in which zoho, zogo, allied to ziohan (as togo to tiuhan), has the old meaning ‘leader.’ Compare ziehen.