An Etymological Dictionary of the German Language/Annotated/ungeheuer
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.