The Collaborative International Dictionary
Let bygones be bygones
Bygone \By"gone`\, n.
Something gone by or past; a past event. ``Let old bygones
be''
--Tennyson.
Let bygones be bygones, let the past be forgotten.
Wiktionary
let bygones be bygones
vb. (context idiomatic English) To ignore or disregard a past offense (when dealing with another individual).
Usage examples of "let bygones be bygones".
Well, that was wartime, and maybe we can let bygones be bygones on account of it was.