The Collaborative International Dictionary of English v.0.48:

Irrevocable \Ir*rev"o*ca*ble\, a. [L. irrevocabilis: cf. F.
   irr['e]vocable. See In- not, and Revoke, and cf.
   Irrevocable.]
   Incapable of being recalled or revoked; unchangeable;
   irreversible; unalterable; as, an irrevocable promise or
   decree; irrevocable fate.
   [1913 Webster]

         Firm and irrevocable is my doom.         --Shak.
   -- Ir*rev"o*ca*ble*ness, n. -- Ir*rev"o*ca*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irrevocable
    adj 1: incapable of being retracted or revoked; "firm and
           irrevocable is my doom"- Shakespeare [syn: irrevocable,
           irrevokable] [ant: revocable, revokable]