The Collaborative International Dictionary of English v.0.48:

Groundless \Ground"less\, a. [AS. grundle['a]s bottomless.]
   Without ground or foundation; wanting cause or reason for
   support; not authorized; false; baseless; as, groundless
   fear; a groundless report, rumor, or assertion. --
   Ground"less*ly, adv. -- Ground"less*ness, n.

WordNet (r) 3.0 (2006):

groundless
    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:

58 Moby Thesaurus words for "groundless":
   baseless, bottomless, built on sand, chimerical, empty, flimsy,
   gratuitous, hypothetical, idle, ill-founded, illusory, imaginary,
   inconclusive, indecisive, not following, not proved,
   not well-founded, speculative, suppositional, tenuous,
   unascertained, unattested, unauthenticated, unbased, uncalled-for,
   uncertified, unconfirmed, uncorroborated, undemonstrated,
   undetermined, unestablished, unfixed, unfounded, ungrounded,
   unjustifiable, unjustified, unproved, unproven, unreasonable,
   unsettled, unshown, unsound, unsubstantiated, unsupportable,
   unsupported, unsupported by evidence, unsustainable, unsustained,
   untenable, untested, untried, unvalidated, unverified, unwarranted,
   vain, without basis, without foundation, without grounds