If you're on a diet but eat one tiny piece of chocolate, it might start an irrevocable slide into bad eating. Describe something as irrevocable if it cannot be undone or taken back.
If you break down irrevocable, you wind up with ir "not," re "back" and vocable from the Latin vocare "to call." So if something is irrevocable, you cannot call it back — it is permanent. You must fulfill an irrevocable promise and live with an irrevocable decision. A law is irrevocable if it states within the law that it cannot be nullified. Now that's final!