An Etymological Dictionary of the German Language/Tadel
Tadel, m., ‘blame, censure, reproof,’ from MidHG. tadel, m. and n., ‘fault, stain, defect (bodily or mental). The word is recorded at a remarkably late period — the end of the 12th cent. — but this, of course, does not prove that it was borrowed. The Teut. root daþ (dad?) contained in it has been compared, probably without just grounds, with Gr. τωθάω (root dhôdh), ‘to deride, mock.’