The Collaborative International Dictionary of English v.0.48:

Unfounded \Un*found"ed\, a.
   1. Not founded; not built or established. --Milton.
      [1913 Webster]

   2. Having no foundation; baseless; vain; idle; as, unfounded
      expectations. --Paley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unfounded
    adj 1: without a basis in reason or fact; "baseless gossip";
           "the allegations proved groundless"; "idle fears";
           "unfounded suspicions"; "unwarranted jealousy" [syn:
           baseless, groundless, idle, unfounded,
           unwarranted, wild]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "unfounded":
   Barmecidal, Barmecide, airy, apparent, apparitional, autistic,
   baseless, bottomless, chimeric, contrary to fact, deceptive,
   delusional, delusionary, delusive, delusory, dereistic, dishonest,
   dreamlike, dreamy, empty, erroneous, fallacious, false, fantastic,
   garbled, gratuitous, groundless, idle, ill-founded, illusional,
   illusionary, illusive, illusory, imaginary, in error, inauthentic,
   inconclusive, indecisive, mendacious, miscited, misleading,
   misquoted, misreported, misstated, not following, not proved,
   not true, ostensible, phantasmagoric, phantasmal, phantom, seeming,
   self-deceptive, self-deluding, specious, spectral, spurious,
   supposititious, truthless, unactual, unascertained, unattested,
   unauthentic, unauthenticated, unauthoritative, unbased,
   uncalled-for, uncertified, unconfirmed, uncorroborated,
   undemonstrated, undetermined, unestablished, unfixed, ungrounded,
   unjustifiable, unjustified, unproved, unproven, unreal, unreliable,
   unsettled, unshown, unsound, unsubstantial, unsubstantiated,
   unsupportable, unsupported, unsupported by evidence, unsustainable,
   unsustained, untenable, untested, untried, untrue, untruthful,
   unvalidated, unverified, unwarranted, vain, visionary,
   without basis, without foundation, without grounds