The Collaborative International Dictionary of English v.0.48:

Questionable \Ques"tion*a*ble\, a.
   1. Admitting of being questioned; inviting, or seeming to
      invite, inquiry. [R.]
      [1913 Webster]

            Thou com'st in such a questionable shape
            That I will speak to thee.            -- Shak.
      [1913 Webster]

   2. Liable to question; subject to be doubted or called in
      question; problematical; doubtful; suspicious.
      [1913 Webster]

            It is questionable whether Galen ever saw the
            dissection of a human body.T.         -- Baker.
      [1913 Webster]

   Syn: Disputable; debatable; uncertain; doubtful;
        problematical; suspicious.
        [1913 Webster]

WordNet (r) 3.0 (2006):

questionable
    adj 1: subject to question; "questionable motives"; "a
           questionable reputation"; "a fire of questionable origin"
           [ant: unquestionable]
    2: able to be refuted [syn: questionable, refutable,
       confutable, confutative]

Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "questionable":
   a bit thick, a bit thin, absurd, ambiguous, amoral, arguable,
   at issue, beguiling, beyond belief, borderline, catchy, confutable,
   conjectural, conscienceless, contestable, controversial,
   controvertible, corrupt, corrupted, criminal, crooked, dark,
   debatable, deceiving, deceptive, delusive, delusory, deniable,
   devious, dishonest, dishonorable, disputable, doubtable, doubtful,
   dubious, dubitable, equivocal, evasive, fallacious, false,
   felonious, fishy, fly-by-night, fraudulent, hallucinatory,
   hard of belief, hard to believe, hardly possible, iffy, ill-got,
   ill-gotten, illusive, illusory, immoral, implausible, improbable,
   in dispute, in doubt, in dubio, in question, inconceivable,
   incredible, indirect, insidious, misleading, mistakable, moot,
   more than doubtful, not deserving belief, not kosher, obscure,
   open to doubt, open to question, open to suspicion, passing belief,
   preposterous, problematic, problematical, refutable, ridiculous,
   rotten, shady, shameless, shifty, sinister, slippery, speculative,
   staggering belief, suppositional, suspect, suspicious, tall, thick,
   thin, trickish, tricksy, tricky, trustless, unbelievable,
   uncertain, unconscienced, unconscientious, unconscionable,
   unconvincing, undependable, underhand, underhanded, unearthly,
   unethical, ungodly, unimaginable, unlikely, unprincipled,
   unpromising, unreliable, unsavory, unscrupulous, unstraightforward,
   unsure, unthinkable, untrustworthy, untrusty, unworthy of belief,
   vague, without remorse, without shame