Let bygones be bygones
Allow the unpleasant things that have happened in the past to be forgotten.
'Let bygones be bygones' is one of the small group of phrases the meaning of which people enquire about more than they do the origin. On the face of it, the meaning is obvious and seems to require no explanation - after all, bygones can hardly be anything other than bygones. We don't have sayings like 'let greengrocers be greengrocers', so is there more to it? As it turns out, there is.
In the 15th century, a bygone was was simply 'a thing that has gone by', i.e. a thing of the past. Shakespeare used it with that meaning in The Winters Tale, 1611:
This satisfaction, The by-gone-day proclaym'd, say this to him.
As time progressed, 'bygones' came to refer specifically to past events that had an unpleasant tinge to them; for example, quarrels or debts. The Scottish churchman Samuel Rutherford recorded that usage of the phrase in a letter during his detention in Aberdeen in 1636. In the letter he regrets the follies of his youth and acknowledges his debt to God in showing him the error of his ways:
"Pray that byegones betwixt me and my Lord may be byegones."
So, there is a little more to the phrase 'let bygones be bygones' than to the more literal 'let sleeping dogs lie' or the old proverb 'let all things past, pass' that was recorded by John Heywood in his 1562 edition of Proverbs. 'Let bygones be bygones' uses both meanings of the word 'bygones' and means, in extended form, 'let the unpleasantness between us become a thing of the past'.