The Collaborative International Dictionary of English v.0.48:

Incontestable \In`con*test"a*ble\, a. [Pref. in- not +
   contestable: cf. F. incontestable.]
   Not contestable; not to be disputed; that cannot be called in
   question or controverted; incontrovertible; indisputable; as,
   incontestable evidence, truth, or facts. --Locke.

   Syn: Incontrovertible; indisputable; irrefragable;
        undeniable; unquestionable; intuitable; certain. --
        In`con*test"a*ble*ness, n. -- In`con*test"a*bly,
        adv.
        [1913 Webster]