An Etymological Dictionary of the German Language/Annotated/abtrünnig

by Friedrich Kluge

abtrünnig, adjective, from the equivalent Middle High German abetrünncc (abetrünne), Old High German abatrunnîg, adjective, ‘recreant’; original sense, ‘he who separates himself from,’ for trennen contains the same stem. Compare also Old High German anttrunno, ‘fugitive,’ Middle High German trünne, ‘a detached troop.’