The Collaborative International Dictionary of English v.0.48:

Undeniable \Undeniable\
   See deniable.

The Collaborative International Dictionary of English v.0.48:

Undeniable \Un`de*ni"a*ble\, a.
   1. Not deniable; incapable of denial; palpably true;
      indisputable; obvious; as, undeniable evidence.
      [1913 Webster]

   2. Unobjectionable; unquestionably excellent; as, a person of
      undeniable connections. [Colloq.] --G. Eliot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undeniable
    adj 1: not possible to deny [ant: deniable]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "undeniable":
   absolute, actual, admitting no question, arrant, authentic,
   classical, complete, confirmable, consummate, crass, de facto,
   decided, definitive, demonstrable, demonstratable, downright,
   egregious, factual, flagrant, for real, genuine, glaring, gross,
   historical, honest-to-God, inappealable, incontestable,
   incontrovertible, indisputable, indubitable, intolerable,
   irrefragable, irrefutable, out-and-out, outright, perfect,
   positive, precious, profound, pronounced, proper, provable, rank,
   real, regular, self-evident, shattering, shocking, stark,
   stark-staring, substantial, superlative, surpassing, testable,
   the veriest, thorough, thoroughgoing, total, true, unanswerable,
   unbearable, unconfutable, unconscionable, unequivocal,
   unimpeachable, unmitigated, unqualified, unquestionable,
   unrefutable, unrelieved, unspoiled, utter, verifiable, veritable