expression
the past is the past; let bygones be bygones
A set phrase used to accept that past events cannot be changed and should be left behind. Often used when forgiving, moving on, or refusing to dwell on old matters.
過去は過去だ。今は前に進もう。
The past is the past. Let's move forward now.
あの失敗はもう過去は過去だから、気にしないで。
That mistake is water under the bridge now, so don't worry about it.
A straightforward phrase composed of 過去 (past) repeated with the topic particle は, literally 'the past is the past'. No complex etymology.