The Collaborative International Dictionary of English v.0.48:

Irrefutable \Ir`re*fut"a*ble\ (?; 277), a. [L. irrefutabilis:
   cf. F. irr['e]futable. See Refute.]
   Incapable of being refuted or disproved; indisputable. --
   Ir`re*fut"a*ble*ness, n. -- Ir`re*fut"a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irrefutable
    adj 1: impossible to deny or disprove; "incontrovertible proof
           of the defendant's innocence"; "proof positive"; "an
           irrefutable argument" [syn: incontrovertible,
           irrefutable, positive]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "irrefutable":
   absolute, actual, adducible, admissible, admitting no question,
   attestative, attestive, authentic, based on, certain,
   circumstantial, conclusive, confirmable, convincing, cumulative,
   damning, decisive, demonstrable, demonstratable, determinative,
   documentary, documented, evidential, evidentiary, ex parte,
   eye-witness, factual, final, firsthand, founded on, grounded on,
   hearsay, historical, implicit, inappealable, incontestable,
   incontrovertible, indicative, indisputable, indubitable,
   irrefragable, irresistible, material, nuncupative, overwhelming,
   presumptive, probative, provable, real, reliable, self-evident,
   significant, suggestive, sure, symptomatic, telling, testable,
   unanswerable, unconfutable, uncontestable, undeniable,
   unimpeachable, unquestionable, unrefutable, valid, verifiable,
   weighty



The Free On-line Dictionary of Computing (30 December 2018):

irrefutable

   The opposite of refutable.