The Collaborative International Dictionary of English v.0.48:

Dubious \Du"bi*ous\, a. [L. dubius, dubiosus, fr. duo two. See
   Two, and cf. Doubt.]
   1. Doubtful or not settled in opinion; being in doubt;
      wavering or fluctuating; undetermined. "Dubious policy."
      --Sir T. Scott.
      [1913 Webster]

            A dubious, agitated state of mind. --Thackeray.
      [1913 Webster]

   2. Occasioning doubt; not clear, or obvious; equivocal;
      questionable; doubtful; as, a dubious answer.
      [1913 Webster]

            Wiping the dingy shirt with a still more dubious
            pocket handkerchief.                  --Thackeray.
      [1913 Webster]

   3. Of uncertain event or issue; as, in dubious battle.

   Syn: Doubtful; doubting; unsettled; undetermined; equivocal;
        uncertain. Cf. Doubtful.
        [1913 Webster]

WordNet (r) 3.0 (2006):

dubious
    adj 1: fraught with uncertainty or doubt; "they were doubtful
           that the cord would hold"; "it was doubtful whether she
           would be admitted"; "dubious about agreeing to go" [syn:
           doubtful, dubious]
    2: open to doubt or suspicion; "the candidate's doubtful past";
       "he has a dubious record indeed"; "what one found uncertain
       the other found dubious or downright false"; "it was more
       than dubitable whether the friend was as influential as she
       thought"- Karen Horney [syn: doubtful, dubious,
       dubitable, in question(p)]
    3: not convinced; "they admitted the force of my argument but
       remained dubious"

Moby Thesaurus II by Grady Ward, 1.0:

177 Moby Thesaurus words for "dubious":
   Humean, Pyrrhonic, a bit thick, a bit thin, absurd, agnostic,
   ambitendent, ambivalent, amoral, arguable, at issue, at loose ends,
   beguiling, beyond belief, capricious, catchy, changeable,
   confutable, conjectural, conscienceless, contestable,
   controversial, controvertible, corrupt, corrupted, criminal,
   crooked, dark, debatable, deceiving, deceptive, delusive, delusory,
   deniable, devious, dishonest, dishonorable, disinclined,
   disputable, distrustful, double-minded, doubtable, doubtful,
   doubting, dubitable, equivocal, evasive, fallacious, false,
   felonious, fence-sitting, fence-straddling, fickle, fishy,
   fly-by-night, fraudulent, from Missouri, hallucinatory,
   hard of belief, hard to believe, hardly possible, hesitant, iffy,
   ill-got, ill-gotten, illusive, illusory, immoral, implausible,
   improbable, in dispute, in doubt, in dubio, in question,
   inconceivable, incredible, incredulous, indecisive, indirect,
   infirm of purpose, insecure, insidious, irresolute, irresolved,
   leery, mercurial, misleading, mistakable, mistrustful, mistrusting,
   moot, more than doubtful, mugwumpian, mugwumpish, mutable,
   not deserving belief, not kosher, of two minds, open,
   open to doubt, open to question, open to suspicion, passing belief,
   preposterous, problematic, questionable, questioning, refutable,
   reluctant, ridiculous, rocky, rotten, scrupulous, shady, shaky,
   shameless, shifty, shy, sinister, skeptic, skeptical, slippery,
   speculative, staggering belief, suppositional, suspect, suspecting,
   suspicious, tall, thick, thin, tottery, treacherous, trickish,
   tricksy, tricky, trustless, unbelievable, uncertain, unclear,
   unconscienced, unconscientious, unconscionable, unconvincing,
   undecided, undependable, underhand, underhanded, undetermined,
   unearthly, unethical, ungodly, unhealthy, unimaginable, unlikely,
   unpredictable, unprincipled, unpromising, unreliable, unresolved,
   unsafe, unsavory, unscrupulous, unsettled, unsound, unstable,
   unsteady, unstraightforward, unsure, unthinkable, untrusting,
   untrustworthy, untrusty, unworthy of belief, wary, without remorse,
   without shame