I. Sources
From the Western Han Dynasty Liu Xiang's "Warring States Policy Chu Ce IV": See rabbits love dogs; It's not too late; Make up later; It is not too late.
It's not too late to call a hound after you see a rabbit. It's never too late to mend.
Second, usage
Usually used as subject, predicate and object in sentences, which means to correct mistakes when you know them.
Example: It doesn't matter if you fail this exam. As the saying goes, "It's not too late to mend."
Extended data
The story of "better late than never";
Once upon a time, there was a herder who kept dozens of sheep, herded them during the day and drove them to a sheepfold surrounded by firewood and stakes at night. One morning, the herdsmen went to herd sheep and found that one sheep was missing. It turned out that there was a hole in the sheepfold. At night, a wolf got in and took a sheep.
Neighbors advised him: "Hurry up and fix the sheepfold and plug the hole." He said, "Why repair the sheepfold when all the sheep are lost?" I didn't accept my neighbor's kind advice. The next morning, he went to herd sheep and found that another sheep was missing. It turned out that the wolf got into the sheepfold from the hole and took away another sheep.
Herdsmen regretted not directly accepting their neighbors' suggestions and taking remedial measures in time. So he quickly blocked the hole, reinforced it as a whole, and made the sheepfold firmly. Since then, the shepherd's sheep have never been grabbed by wolves.