The Collaborative International Dictionary of English v.0.48:

Stultify \Stul"ti*fy\, v. t. [imp. & p. p. Stultified; p. pr.
   & vb. n. Stultifying.] [L. stultus foolish + -fy.]
   1. To make foolish; to make a fool of; as, to stultify one by
      imposition; to stultify one's self by silly reasoning or
      conduct. --Burke.
      [1913 Webster]

   2. To regard as a fool, or as foolish. [R.]
      [1913 Webster]

            The modern sciolist stultifies all understanding but
            his own, and that which he conceives like his own.
      --Hazlitt.
      [1913 Webster]

   3. (Law) To allege or prove to be of unsound mind, so that
      the performance of some act may be avoided.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stultify
    v 1: prove to be of unsound mind or demonstrate someone's
         incompetence; "nobody is legally allowed to stultify
         himself"
    2: cause to appear foolish; "He stultified himself by
       contradicting himself and being inconsistent"
    3: deprive of strength or efficiency; make useless or worthless;
       "This measure crippled our efforts"; "Their behavior
       stultified the boss's hard work" [syn: cripple, stultify]