An Etymological Dictionary of the German Language/Annotated/klug

by Friedrich Kluge

klug, adjective, ‘knowing, prudent, shrewd,’ from Middle High German kluoc (g), ‘fine, pretty, tender, superb, brave, polite, prudent, sly’; in Old High German not recorded, whether by chance or no is not known. It is thought to have been borrowed from Low German, although the word in the non-High German languages has a final k, Low German klôk, Dutch klock, ‘prudent, brave, great, corpulent’ (not found in English; Scandinavian klókr, ‘prudent, cunning,’ is supposed to be a German loan-word). No clue to an etymological explanation of the adjective can be discovered.