The Collaborative International Dictionary of English v.0.48:

Deniable \De*ni"a*ble\, a. [See Deny.]
   Capable of being, or liable to be, denied.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deniable
    adj 1: capable of being denied or contradicted [ant:
           undeniable]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "deniable":
   arguable, at issue, confutable, conjectural, contestable,
   controversial, controvertible, debatable, 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