The Collaborative International Dictionary of English v.0.48:

Indemonstrable \In`de*mon"stra*ble\, a. [L. indemonstrabilis.
   See In- not, and Demonstrable.]
   Incapable of being demonstrated. --
   In`de*mon"stra*ble*ness, n.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

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