the past; something that happened in the past that you would like to be forgotten.
Why can't you let bygones be bygones?
An expression meant to pre-emptively defer an argument based on an opposing viewpoint. It signals to the other person that you already understand their position and accept that you disagree with each other. It is a reference to the phrase "let bygones be bygones," and was popularized by the TV show "Ally McBeal."
Sorry, I think socialized medicine is a good idea. Bygones!
(n.) meaning something in the past
Let's let bygones be bygones.