what's done is done

There is no changing something; it's finished or final. For example, I forgot to include my dividend income in my tax return but what's done is done—I've already mailed the form. This expression uses done in the sense of “ended” or “settled,” a usage dating from the first half of the 1400s.

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