Learn English – What does the phrase “The horse has left the barn” mean

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What does the phrase "The horse has left the barn" mean?

I'm asking this question because I was watching the live testimony of Acting Director of National Intelligence Joseph Maguire before a congressional committee and he said this phrase at the end of the meeting. When asked by the chairman of the committee should the investigation on Donald Trump be conducted he replied:

"The horse has left the barn. You have all of the information. You
have the whistleblower complaint. You have the letter from the ICIG.
You have the Office of Legal Counsel opinion and you have the
transcript from the president".

I tried to look this up, but it doesn't seem that this is an established idiom. In my opinion, the phrase can mean that it's too late to ask whether one should conduct the investigation because too many documents have been made public.

I'd be thankful for a few examples that'd help me to understand the meaning of this phrase (or idiom).

Best Answer

The exact phrasing given by OP is relatively uncommon 1 compared to what I see as the two relevant "idiomatic standard" usages...

1: to close the stable door after the horse has bolted
to have tried to prevent something happening, but to have done so too late to prevent damage being done
...and...
2: That ship has already left port / sailed
that chance is now gone; it is too late

Note that although those two definitions are similar, they're not generally considered "interchangeable". #1 essentially draws attention to the fact that a potential remedial action is no longer appropriate (because the thing it was supposed to prevent has already happened), whereas #2 is about having missed a chance (it's now too late to take advantage of some favourable opportunity).

OP's version looks to me like a "mash-up" of those two idiomatic usages.

1 I searched for the two following text strings in Google Books...

after the horse had bolted (23 hits)
after the horse had left the barn (8 hits)

I think the ratio is even more extreme with Simple Past (the horse has bolted / left), but you have to scroll through several more pages of results to get to the final totals for those, and I suspect Google Books becomes less "accurate" in such situations.

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