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]