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]

WordNet (r) 3.0 (2006):

incontestable
    adj 1: incapable of being contested or disputed [syn:
           incontestable, incontestible] [ant: contestable]
    2: not open to question; obviously true; "undeniable guilt";
       "indisputable evidence of a witness" [syn: incontestable,
       indisputable, undisputable]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "incontestable":
   absolute, actual, admitting no question, certain, confirmable,
   demonstrable, demonstratable, factual, historical, impenetrable,
   impregnable, inappealable, incontrovertible, indisputable,
   indomitable, indubitable, inexpugnable, insuperable, invincible,
   inviolable, invulnerable, irrefragable, irrefutable, irresistible,
   overpowering, overwhelming, provable, real, resistless,
   self-evident, sure, testable, unanswerable, unassailable,
   unbeatable, unconfutable, unconquerable, uncontestable, undeniable,
   unimpeachable, unquestionable, unrefutable, unsubduable,
   unsurmountable, unyielding, verifiable