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]