The Collaborative International Dictionary of English v.0.48:

Arguable \Ar"gu*a*ble\, a.
   Capable of being argued; admitting of debate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arguable
    adj 1: capable of being supported by argument
    2: open to argument or debate; "that is a moot question" [syn:
       arguable, debatable, disputable, moot]

Moby Thesaurus II by Grady Ward, 1.0:

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