An Etymological Dictionary of the German Language/Ohn-

by Friedrich Kluge

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