The Collaborative International Dictionary of English v.0.48:

Doubtful \Doubt"ful\, a.
   1. Not settled in opinion; undetermined; wavering; hesitating
      in belief; also used, metaphorically, of the body when its
      action is affected by such a state of mind; as, we are
      doubtful of a fact, or of the propriety of a measure.
      [1913 Webster]

            Methinks I should know you, and know this man;
            Yet I am doubtful.                    --Shak.
      [1913 Webster]

            With doubtful feet and wavering resolution.
                                                  --Milton.
      [1913 Webster]

   2. Admitting of doubt; not obvious, clear, or certain;
      questionable; not decided; not easy to be defined,
      classed, or named; as, a doubtful case, hue, claim, title,
      species, and the like.
      [1913 Webster]

            Beauty is but a vain and doubtful good. --Shak.
      [1913 Webster]

            Is it a great cruelty to expel from our abode the
            enemy of our peace, or even the doubtful friend [i.
            e., one as to whose sincerity there may be doubts]?
                                                  --Bancroft.
      [1913 Webster]

   3. Characterized by ambiguity; dubious; as, a doubtful
      expression; a doubtful phrase.
      [1913 Webster]

   4. Of uncertain issue or event.
      [1913 Webster]

            We . . . have sustained one day in doubtful fight.
                                                  --Milton.
      [1913 Webster]

            The strife between the two principles had been long,
            fierce, and doubtful.                 --Macaulay.
      [1913 Webster]

   5. Fearful; apprehensive; suspicious. [Obs.]
      [1913 Webster]

            I am doubtful that you have been conjunct
            And bosomed with her.                 --Shak.

   Syn: Wavering; vacillating; hesitating; undetermined;
        distrustful; dubious; uncertain; equivocal; ambiguous;
        problematical; questionable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

doubtful
    adj 1: 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)]
    2: 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]
    3: unsettled in mind or opinion; "drew a few tentative
       conclusions" [syn: doubtful, tentative]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "doubtful":
   Humean, Pyrrhonic, a bit thick, a bit thin, absurd, agnostic,
   ambiguous, amoral, arguable, at issue, beyond belief, borderline,
   chancy, clouded, confutable, conjectural, conscienceless,
   contestable, contingent, controversial, controvertible, corrupt,
   corrupted, criminal, crooked, dark, debatable, deniable, devious,
   dishonest, dishonorable, disputable, disreputable, distrustful,
   doubtable, doubting, dubious, dubitable, equivocal, evasive,
   felonious, fishy, fraudulent, from Missouri, hard of belief,
   hard to believe, hardly possible, hazy, hesitant, hesitating, iffy,
   ill-got, ill-gotten, immoral, implausible, improbable, in dispute,
   in doubt, in dubio, in question, inconceivable, incredible,
   incredulous, indecisive, indefinite, indeterminate, indirect,
   insecure, insidious, leery, mistakable, mistrustful, mistrusting,
   moot, more than doubtful, not deserving belief, not kosher,
   obscure, open, open to doubt, open to question, open to suspicion,
   passing belief, precarious, preposterous, problematic, queasy,
   questionable, questioning, refutable, ridiculous, rocky, rotten,
   sceptical, scrupulous, shady, shaky, shameless, shifty, shy,
   sinister, skeptic, skeptical, slippery, speculative,
   staggering belief, suppositional, suspect, suspecting, suspicious,
   tall, thick, thin, tottery, touch-and-go, treacherous, tricky,
   unbelievable, uncertain, unclear, unconscienced, unconscientious,
   unconscionable, unconvinced, unconvincing, undecided, undependable,
   underhand, underhanded, unearthly, uneasy, unethical, ungodly,
   unhealthy, unimaginable, unlikely, unpredictable, unprincipled,
   unpromising, unreliable, unresolved, unsafe, unsavory,
   unscrupulous, unsettled, unsound, unstable, unsteady,
   unstraightforward, unsure, unthinkable, untrusting, untrustworthy,
   unworthy of belief, vacillating, vague, wary, without remorse,
   without shame