Don’t waste time taking precautions when the damage has already been done.

The advice ‘don’t shut the stable door…’ is one of the most long standing English proverbs. A form of it is found in 1390 John Gower’s enormously long Middle English poem Confessio Amantis. This was published in 1390 and it may be that the proverb was in use in everyday language for some time before that:

For whan the grete Stiede Is stole, thanne he taketh hiede, And makth the stable dore fast.

It appears again in John Heywood’s 1546 collection A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue:

To late this repentance shewd is.
Whan the stede is stolne, shut the stable durre.