An Etymological Dictionary of the German Language/Annotated/ungeheuer
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ungeheuer, adjective, ‘monstrous, atrocious,’ from Middle High German ungehiure, Old High German ungihiuri, ‘uncanny, frightful’; allied to Angeheuer, neuter, ‘monster,’ from Middle High German ungehiure, ‘savage, dragon, ghostly creature,’ Old High German ungihiurì, ‘monster.’ See geheuer.