The Collaborative International Dictionary of English v.0.48:

Illegitimate \Il`le*git"i*mate\, v. t. [imp. & p. p.
   Illegitimated; p. pr. & vb. n. Illegitimating.]
   To render illegitimate; to declare or prove to be born out of
   wedlock; to bastardize; to illegitimatize.
   [1913 Webster]

         The marriage should only be dissolved for the future,
         without illegitimating the issue.        --Bp. Burnet.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Illegitimate \Il`le*git"i*mate\, a.
   1. Not according to law; not regular or authorized; unlawful;
      improper.
      [1913 Webster]

   2. Unlawfully begotten; born out of wedlock; bastard; as, an
      illegitimate child.
      [1913 Webster]

   3. Not legitimately deduced or inferred; illogical; as, an
      illegitimate inference.
      [1913 Webster]

   4. Not authorized by good usage; not genuine; spurious; as,
      an illegitimate word.
      [1913 Webster]

   Illegitimate fertilization, or Illegitimate union (Bot.),
      the fertilization of pistils by stamens not of their own
      length, in heterogonously dimorphic and trimorphic
      flowers. --Darwin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

illegitimate
    adj 1: contrary to or forbidden by law; "an illegitimate seizure
           of power"; "illicit trade"; "an outlaw strike"; "unlawful
           measures" [syn: illegitimate, illicit, outlaw(a),
           outlawed, unlawful]
    2: of marriages and offspring; not recognized as lawful [ant:
       legitimate]
    n 1: the illegitimate offspring of unmarried parents [syn:
         bastard, by-blow, love child, illegitimate child,
         illegitimate, whoreson]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "illegitimate":
   actionable, affected, against the law, anarchic, anarchistic,
   anomic, apocryphal, artificial, assumed, bantling, bar sinister,
   baseborn, bastard, bastard child, bastardy, black-market, bogus,
   bootleg, brummagem, by-blow, chargeable, colorable, colored,
   contraband, contrary to law, counterfeit, counterfeited, criminal,
   distorted, dressed up, dummy, embellished, embroidered, ersatz,
   factitious, fake, faked, false, falsified, fatherless, feigned,
   felonious, fictitious, fictive, flawed, garbled, illegal,
   illegitimacy, illegitimate child, illicit, imitation,
   impermissible, improper, incorrect, invalid, irregular, junky,
   justiciable, lawless, love child, make-believe, man-made,
   misbegotten, miscreated, mock, natural, nonconstitutional,
   nonlegal, nonlicit, outlaw, outlawed, perverted, phony, pinchbeck,
   pretended, pseudo, punishable, put-on, quasi, queer, self-styled,
   sham, shoddy, simulated, so-called, soi-disant, spurious,
   supposititious, synthetic, tin, tinsel, titivated, triable,
   twisted, unallowed, unauthentic, unauthorized, unconstitutional,
   under-the-counter, under-the-table, ungenuine, unlawful, unnatural,
   unofficial, unreal, unstatutory, unwarrantable, unwarranted,
   warped, wrongful



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ILLEGITIMATE. That which is contrary to law; it is usually applied to
children born out of lawful wedlock. A bastard is sometimes called an
illegitimate child.