The Collaborative International Dictionary of English v.0.48:

Irrefragable \Ir*ref"ra*ga*ble\, a. [F. irr['e]fragable, L.
   irrefragabilis. See Refragable.]
   Not refragable; not to be gainsaid or denied; not to be
   refuted or overthrown; irrefutable; unanswerable;
   incontestable; undeniable; as, an irrefragable argument;
   irrefragable evidence. -- Ir*ref"ra*ga*ble*ness, n. --
   Ir*ref"ra*ga*bly, adv.

   Syn: Incontrovertible; unanswerable; indisputable;
        unquestionable; incontestable; indubitable; undeniable;
        irrefutable.
        [1913 Webster]