Source : Webster's Revised Unabridged Dictionary (1913)

Irrevokable \Ir`re*vok"a*ble\, a. [Pref. ir- not + revoke.]
   Irrevocable. [R.]

Source : WordNet®

irrevokable
     adj : impossible to retract or revoke; "firm and irrevocable is my
           doom"- Shakespeare [syn: {irrevocable}] [ant: {revocable}]