An Etymological Dictionary of the German Language/Annotated/Keusch
keusch, adjective, ‘chaste, pure,’ from Middle High German kiusche, kiusch, adjective, ‘moderate, quiet, modest, bashful'; Old High German chûski, adjective, ‘continent, moderate.’ Anglo-Saxon cûse is borrowed from the Old Saxon of the Heliand, Old Saxon *kûsci, of which only the corresponding adverb cûsco is recorded; Dutch kuisch, ‘cleanly, chaste.’ The primary meaning of the Old Teutonic adjective, which appears in all these forms, is presumably ‘pure’; compare Dutch kuischen, ‘to clean, purify’; Old High German unchûskî, ‘dirt’ (also Hessian unkeuscher Weg, ‘road in bad condition’). —