The Collaborative International Dictionary of English v.0.48:

Unquestionable \Un*ques"tion*a*ble\, a.
   1. Not questionable; as, an unquestionable title.
      [1913 Webster]

   2. Not inviting questions or conversation. [R.] --Shak.
      [1913 Webster] -- Un*ques"tion*a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unquestionable
    adj 1: incapable of being questioned; "unquestionable authority"
           [ant: questionable]
    2: not counterfeit or copied; "an authentic signature"; "a bona
       fide manuscript"; "an unquestionable antique"; "photographs
       taken in a veritable bull ring" [syn: authentic, bona
       fide, unquestionable, veritable]
    3: not open to question; "an unquestionable (or unequivocal)
       loss of prestige"

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "unquestionable":
   absolute, actual, admitting no question, ascertained, attested,
   authenticated, believable, bona fide, categorically true, certain,
   certified, clear, colorable, conceivable, conclusive, confirmable,
   confirmed, corroborated, credible, deep-seated, definite,
   demonstrable, demonstratable, demonstrated, dependable, determined,
   documentary, effectual, esoteric, established, factual, fiduciary,
   flat, genuine, historical, immanent, implanted, implicit,
   inalienable, inappealable, incontestable, incontrovertible,
   indisputable, indubitable, indwelling, infixed, ingrained,
   inherent, inner, internal, intrinsic, inward, inwrought,
   irreducible, irrefragable, irrefutable, manifest, not in error,
   objectively true, obvious, patent, plausible, positive, private,
   provable, proved, real, reliable, resident, secret, self-evident,
   subjective, substantiated, sure, sure-enough, tenable, testable,
   true, true as gospel, trustworthy, trusty, truthful, unalienable,
   unanswerable, unchallengeable, unconfutable, unconfuted,
   uncontestable, undeniable, undenied, undoubted, unequivocal,
   unerroneous, unexceptionable, unfallacious, unfalse, unimpeachable,
   unmistakable, unmistaken, unrefutable, unrefuted, up-and-down,
   validated, veracious, verifiable, verified, veritable, very,
   well-founded, well-grounded, worthy of faith