The Collaborative International Dictionary of English v.0.48:

Speculative \Spec"u*la*tive\ (sp[e^]k"[-u]*l[.a]*t[i^]v), a.
   [Cf. F. sp['e]culatif, L. speculativus.]
   1. Given to speculation; contemplative.
      [1913 Webster]

            The mind of man being by nature speculative.
                                                  --Hooker.
      [1913 Webster]

   2. Involving, or formed by, speculation; ideal; theoretical;
      not established by demonstration. --Cudworth.
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   3. Of or pertaining to vision; also, prying; inquisitive;
      curious. [R.] --Bacon.
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   4. Of or pertaining to speculation in land, goods, shares,
      etc.; as, a speculative dealer or enterprise.
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            The speculative merchant exercises no one regular,
            established, or well-known branch of business. --A.
                                                  Smith.
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   5. (Finance) More risky than typical investments; not
      investment grade.
      [PJC] -- Spec"u*la*tive*ly, adv. --
      Spec"u*la*tive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

speculative
    adj 1: not financially safe or secure; "a bad investment"; "high
           risk investments"; "anything that promises to pay too
           much can't help being risky"; "speculative business
           enterprises" [syn: bad, risky, high-risk,
           speculative]
    2: not based on fact or investigation; "a notional figure of
       cost helps in determining production costs"; "speculative
       knowledge" [syn: notional, speculative]
    3: showing curiosity; "if someone saw a man climbing a light
       post they might get inquisitive"; "raised a speculative
       eyebrow" [syn: inquisitive, speculative, questioning,
       wondering(a)]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "speculative":
   abstract, academic, adventurous, aleatory, analytical, arguable,
   armchair, at issue, chancy, closet, cogitative, cognitive,
   concentrating, concentrative, conceptive, conceptual,
   conceptualized, confutable, conjectural, contemplating,
   contemplative, contestable, controversial, controvertible, curious,
   debatable, deliberating, deliberative, deniable, dicey, disputable,
   doubtable, doubtful, dubious, dubitable, excogitating,
   full of risk, hazardous, hypothetic, hypothetical, ideal,
   idealistic, idealized, ideational, ideative, iffy, impractical,
   in dispute, in doubt, in dubio, in question, inquiring,
   intellectual, introspective, meditating, meditative, mental,
   mistakable, moot, museful, musing, noetic, notional, open to doubt,
   open to question, pensive, pondering, postulatory, prehensive,
   problematic, questionable, questioning, ratiocinative, rational,
   reflecting, reflective, refutable, riskful, risky, ruminant,
   ruminating, ruminative, serious, sober, suppositional,
   supposititious, suppositive, suspect, suspicious, theoretical,
   thinking, thought, thoughtful, uncertain, unpractical, unproved,
   unproven, unrealistic, unreliable, untested, untrustworthy,
   venturesome, venturous, wildcat, wistful