The Collaborative International Dictionary of English v.0.48:

Ambiguous \Am*big"u*ous\, a. [L. ambiguus, fr. ambigere to
   wander about, waver; amb- + agere to drive.]
   Doubtful or uncertain, particularly in respect to
   signification; capable of being understood in either of two
   or more possible senses; equivocal; as, an ambiguous course;
   an ambiguous expression.
   [1913 Webster]

         What have been thy answers? What but dark,
         Ambiguous, and with double sense deluding? --Milton.
   [1913 Webster]

   Syn: Doubtful; dubious; uncertain; unsettled; indistinct;
        indeterminate; indefinite. See Equivocal.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ambiguous
    adj 1: open to two or more interpretations; or of uncertain
           nature or significance; or (often) intended to mislead;
           "an equivocal statement"; "the polling had a complex and
           equivocal (or ambiguous) message for potential female
           candidates"; "the officer's equivocal behavior increased
           the victim's uneasiness"; "popularity is an equivocal
           crown"; "an equivocal response to an embarrassing
           question" [syn: equivocal, ambiguous] [ant:
           unambiguous, unequivocal, univocal]
    2: having more than one possible meaning; "ambiguous words";
       "frustrated by ambiguous instructions, the parents were
       unable to assemble the toy" [ant: unambiguous]
    3: having no intrinsic or objective meaning; not organized in
       conventional patterns; "an ambiguous situation with no frame
       of reference"; "ambiguous inkblots"

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "ambiguous":
   Greek to one, agnostic, amalgamated, ambivalent, amphibious,
   amphibolic, amphibological, amphibolous, antinomic,
   beyond understanding, blended, capricious, chancy, changeable,
   combined, complex, composite, compound, compounded, confusable,
   conglomerate, cryptic, dappled, dicey, double-faced, doubtful,
   doubting, dubious, eclectic, enigmatic, enigmatical, equivocal,
   equivocatory, erratic, fickle, fifty-fifty, fishy, foggy,
   half-and-half, hesitant, hesitating, heterogeneous, impenetrable,
   inarticulate, incalculable, incognizable, incoherent,
   incomprehensible, inconclusive, indecisive, indefinite,
   indemonstrable, indeterminate, indiscriminate, indistinct,
   inscrutable, intricate, ironic, irresolute, jumbled, many-sided,
   medley, mingled, miscellaneous, misleading, misty, mixed, motley,
   multifaceted, multinational, multiracial, multivocal, mysterious,
   numinous, obscure, opaque, oracular, oxymoronic, paradoxical,
   past comprehension, patchy, pluralistic, polysemantic, polysemous,
   problematic, promiscuous, puzzling, questionable, rambling,
   scrambled, self-contradictory, skeptical, suspect, syncretic,
   tenebrous, thrown together, touch-and-go, unaccountable, uncertain,
   unclear, unconfirmable, unconnected, unconvinced, undefined,
   undependable, undivinable, unfathomable, unforeseeable,
   unintelligible, unknowable, unpersuaded, unpredictable, unprovable,
   unreliable, unsearchable, unsettled, unsure, unverifiable, vague,
   variable, varied, wavering, whimsical