1
general::
An irrevocable agreement, as in Once you've signed the lease, it's a done deal. This slangy expression, first recorded in 1979, may have come from done thing, originating in the late 1600s.
American Heritage Idioms
2
general::
Phrase(s): a done deal
a completed deal; something that is settled. • It’s too late. It’s a done deal. • The sale of the property is a done deal. There is nothing that can be done now.
McGrawhill's American Idioms And Phrasal Verbs