I don't know the real origin of this phrase, but I will tell you what I feel when I hear this English proverb. I have lots of farmers in my family, and they have lots of horses and cows. How do you stop a horse from leaving the barn? Well, you could close the barn doors, or lock the horse in it's pen, or you can tie the halter to a post. These are all good solutions. But none of those "solutions" mean anything if "the horse is already out of the barn". So, in my mind, the phrase means that you may have a good idea, but it is too late to do any good. For example, here is a news article... https://www.thetelegram.com/business/local-business/st-johns-hotels-look-for-better-days-in-tough-economy-391469/ This is a story about how AirBnB rentals are taking business away from traditional hotels. "The thing is, the horse is out of the barn when it comes to Airbnbs. You're not going to stop that, and I don't know if you should," he said. "But I think it should definitely be regulated and taxed like traditional hoteliers are. I think that would make it fair." Perhaps it would have been a good idea to have a system in place to "regulate and tax" AirBnB's BEFORE they started opening for business, but that didn't happen. So now they are trying to find a solution when "the horse is already out of the barn".

It means that it’s too late now to change what’s happened. It’s an older metaphor that comes from the days when horses were used for transportation, especially to pull vehicles like stagecoaches and carriages. When the horse has left the barn, it’s started its journey and you’re too late to get on it. Another metaphor that means the same is, “That train has left the station.”

The meaning is "The damage is done." Something has happened and should have been prevented, but now it is too late.

The meaning is "The damage is done." Something has happened and should have been prevented, but now it is too late.