The Collaborative International Dictionary of English v.0.48:

Debatable \De*bat"a*ble\, a. [Cf. OF. debatable. See Debate.]
   Liable to be debated; disputable; subject to controversy or
   contention; open to question or dispute; as, a debatable
   question.
   [1913 Webster]

   The Debatable Land or or Ground, a tract of land between
      the Esk and the Sark, claimed by both England and
      Scotland; the Batable Ground.
      [1913 Webster]

WordNet (r) 3.0 (2006):

debatable
    adj 1: open to doubt or debate; "If you ever get married, which
           seems to be extremely problematic" [syn: debatable,
           problematic, problematical]
    2: open to argument or debate; "that is a moot question" [syn:
       arguable, debatable, disputable, moot]
    3: capable of being disproved [syn: debatable, disputable]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "debatable":
   arguable, at issue, confutable, conjectural, contestable,
   controversial, controvertible, deniable, disputable, doubtable,
   doubtful, dubious, dubitable, iffy, in dispute, in doubt, in dubio,
   in question, mistakable, moot, open to doubt, open to question,
   polemic, polemical, problematic, problematical, questionable,
   refutable, speculative, suppositional, suspect, suspicious,
   uncertain, undecided, unsettled, unsure