An Etymological Dictionary of the German Language/Annotated/schwank
schwank, adjective, ‘staggering, insteady,’ from Middle High German swanc (k), adjective, ‘pliant, thin, slender’; so too the equivalent Middle High German kel, Anglo-Saxon swgneor, Old Icelandic svang-r; allied to the root swing, in schwingen; hence schwant is literally ‘easily swung, pliant.’ With the Teutonic cognates Italian sguancio, ‘wryness,’ has been connected. .