Late Middle English. Originally probably from un- + revoke + -able, perhaps after classical Latin irrevocābilisirrevocable; compare slightly later revocable.
Definition of unrevocable in US English:
unrevocable
adjectiveʌnˈrɛvəkəb(ə)l
Scottish rare
Irrevocable.
Origin
Late Middle English. Originally probably from un- + revoke + -able, perhaps after classical Latin irrevocābilis irrevocable; compare slightly later revocable.