1
general::
Phrase(s): What’s done is done.
Prov. It is final and in the past. • It’s too late to change it now. What’s done is done. • What’s done is done. The past cannot be altered.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms