What does let gone be by gone mean?

Excuse me, could you please clarify the meaning of the phrase "let gone be by gone"? It seems to be a somewhat unusual expression, and I'm having trouble grasping its full significance. Is it a way of saying to forget about something that has already happened and move on, or does it have a more specific connotation that I'm missing? I'd appreciate it if you could provide some context or an example to help me better understand the phrase.