This idiom was a sentence excerpted from one masterpiece written by Su Shi (sū shì 苏轼),who estabished himself as a renowned poet in The Song Dynasty (sòng cháo 宋朝,960—1279 AD).

Lü Meng (lǚ méng 吕蒙) was a meritorious general of the State of Wu during the Three Kingdoms Period (sān guó 三国时代,220-280AD). He came from a poor family and had not the chance to go to school when he was young. When he became a general, the duke of Wu encouraged him to read some books. Lü Meng took his advice, and stared to study hard. Even when he was marching or fighting, he would find time to study. There was always a book in his hand. Finally, Lü Meng became a learned general.
This idiom is used to describe being diligent in study.

In the late stage of the Three Kingdoms period (sān guó 三国), Sima Yan (sī mā yān 司马炎), the ruler of the state of Jin ( jìn guó 晋国), conquered the Kingdom of Shu (shǔguó 蜀国) and then took control of the Kingdom of Wei (wèi guó 魏国). However, Sima Yan was a man great ambition .Soon after the previous successes, he began to plan to put the Kingdom of Wu (wú guó 吴国) into his pocket so as to unify China as a whole. He called his civil and military officials to discuss the plan to destroy the Kingdom of Wu. Most of them thought that it would be difficult to try to destroy the Kingdom of Wu with one action because the Kingdom of Wu was still powerfull. So it would be better to wait until fuller preparations were made.

Tao Yuanming (táo yuān míng 陶渊明), a famous writer of the Eastern Jin Dynasty (dōng jìn 东晋,317-420AD), wrote the well-known essay Peach-Blossom Spring (táo huā yuán jì 桃花源记).

Chen Yau-tz, of the Sung dynasty (sòng cháo 宋朝), was a very talented archer. He hit the bull's eye every time, and so everyone called him "The Magic Archer." Chen Yau-tz felt that he was the best archer in the world, and so he was very proud.