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.
Words nearby done deal
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.