The legs of a lame person dangle uselessly, they carry no “weight” (authority), and are unable to carry him where he wants to go. Similarly, if a fool does speak a proverb, trying to act wise, it “dangles” uselessly from his lips. It has no authority, because the speaker has no authority. Furthermore, it was likely spoken inappropriately. In any case, like the lame leg, it will not get him where he intended to go.