The phrase 'Let bygones be bygones' originated in the 15th century.
A good example was recorded in a letter by Scottish churchman Samuel Rutherford, acknowledging the follies of his youth:
“Pray that byegones betwixt me and my Lord may be byegones.”
* betwixt =between.
To ‘let bygones be bygones’ is to allow the unpleasant things that have happened in the past be forgotten.
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What's the origin of the phrase 'Let bygones be bygones'?
‘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’, that is, a thing of the past. Shakespeare used it with that meaning in The Winters Tale, 1611:
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:
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 A Dialogue conteinyng the nomber in effect of all the Prouerbes in the Englishe tongue. ‘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’.