let bygones be bygones
to forget something bad that someone has done to you and forgive them

[TahlilGaran] Dictionary of Contemporary English ▲

let bygones be bygones
forget about unpleasant things that have happened in the past.
There is a feeling here that we should let bygones be bygones and move on to more important things.

Usage notes: sometimes used in the form bygones are bygones:
What is past history is history, what is done is finished, and bygones are bygones.

[TahlilGaran] English Idioms Dictionary ▲

Let bygones be bygones.
[slightly formal]
something that you say in order to tell someone to forget about unpleasant things that have happened in the past.
Why can't you put all that bad feeling behind you and let bygones be bygones?

[TahlilGaran] English Idioms Dictionary ▲

let bygones be bygones
forget about problems that happened in the past
"We need to let bygones be bygones and forget about our past differences."

My aunt decided to let bygones be bygones and she finally decided to talk to my mother again.

[TahlilGaran] English Idioms Dictionary ▲

let bygones be bygones
v. phr. To let the past be forgotten.
After a long, angry quarrel the two boys agreed to let bygones be bygones and made friends again.
We should let bygones be bygones and try to get along with each other.
Synonym: FORGIVE AND FORGET.
Compare: BURY THE HATCHET, LIVE AND LET LIVE.

[TahlilGaran] English Idioms Dictionary ▲