An Etymological Dictionary of the German Language/Annotated/Ohn-

by Friedrich Kluge

Ohn- in Ohnmacht is due to the attempt to assign a more intelligible meaning to Omacht, which originated in Old High German and Middle High German â-maht; the prefix ô from the old â had become obscure in the compound. With regard to Old High German â, ‘un-,’ compare â-teili, ‘having no share in,’ Middle High German â-sętze, ‘unoccupied,’ Anglo-Saxon œ̂-men, ‘devoid of men.’ —