An Etymological Dictionary of the German Language/Annotated/schwach
schwach, adjective, ‘weak, infirm, feeble,’ from Middle High German swach, adjective, ‘low, poor, despised, weak, infirm’; wanting in Old High German as well as in the other Teutonic dialects. The usual derivation from a Teutonic root swek, ‘to swell’ (schwach, originally ‘that which has lost its savour’), must be abandoned; schwach is rather allied to siech, so that the Teutonic roots suk, swak, are to be assumed (compare the following word).