An Etymological Dictionary of the German Language/klug
klug, adj., ‘knowing, prudent, shrewd,’ from MidHG. kluoc (g), ‘fine, pretty, tender, superb, brave, polite, prudent, sly’; in OHG. not recorded, whether by chance or no is not known. It is thought to have been borrowed from LG., although the word in the non-HG. languages has a final k, LG. klôk, Du. klock, ‘prudent, brave, great, corpulent’ (not found in E.; Scand. klókr, ‘prudent, cunning,’ is supposed to be a G. loan-word). No clue to an etymological explanation of the adj. can be discovered.