The Collaborative International Dictionary of English v.0.48:

Bastardize \Bas"tard*ize\, v. t. [imp. & p. p. Bastardized
   (?); p. pr. & vb. n. Bastardizing.]
   1. To make or prove to be a bastard; to stigmatize as a
      bastard; to declare or decide legally to be illegitimate.
      [1913 Webster]

            The law is so indulgent as not to bastardize the
            child, if born, though not begotten, in lawful
            wedlock.                              --Blackstone.
      [1913 Webster]

   2. To beget out of wedlock. [R.] --Shak.
      [1913 Webster]

   3. to change something (for example, art forms) so that its
      value declines; to debase.

   Syn: bastardise.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bastardize
    v 1: change something so that its value declines; for example,
         art forms [syn: bastardize, bastardise]
    2: declare a child to be illegitimate [syn: bastardize,
       bastardise]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "bastardize":
   adulterate, brutalize, contaminate, corrupt, cut, debase, debauch,
   demoralize, denaturalize, denature, deprave, dilute, doctor,
   doctor up, fortify, lace, pervert, pollute, spike, tamper with,
   vitiate, warp, water, water down