A fait accompli is something that's already done. If your dad warns you not to play an April Fool's Day trick on your sister, but you've already put a giant inflatable Godzilla in the shower, that's a fait accompli.
The noun fait accompli, pronounced "fate uh-COM-plee," describes something that has already happened. It often refers to a change or decision made by some authority on behalf of the people who will actually be affected. If workers continue to strike after a change in their working conditions has taken effect, they're protesting a fait accompli. The phrase fait accompli is French, and it literally means "an accomplished fact."