The Collaborative International Dictionary of English v.0.48:

Unprovable \Unprovable\
   See provable.

WordNet (r) 3.0 (2006):

unprovable
    adj 1: not provable; "it was both unproved and unprovable"

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "unprovable":
   agnostic, ambiguous, capricious, chancy, changeable,
   controvertible, dicey, doubting, equivocal, erratic, fickle,
   hesitant, hesitating, incalculable, indecisive, indemonstrable,
   irresolute, polysemous, skeptical, touch-and-go, unaccountable,
   unattestable, uncertain, unconfirmable, unconvinced,
   undemonstrable, undemonstratable, undivinable, unforeseeable,
   unpersuaded, unpredictable, unsubstantiatable, unsupportable,
   unsure, unsustainable, unverifiable, variable, wavering,
   whimsical