Bygones are old hurts or old fights — things in the past that still sting. In real life you almost only meet this word in one phrase: let bygones be bygones. That means let’s stop being angry about what happened, forgive the other person, and start again. People say it after a quarrel, in families, at work, even between countries. It is a little careful and a little kind; it is not a joke. Though actually you almost never see the word outside that one phrase: you do not usually talk about “a bygone” or “many bygones” on their own in everyday speech.

既往不咎（幾乎只出現在 let bygones be bygones）