General Dou Ying, a nephew of Empress Dowager Dou, was appointed by Emperor Jing of Han to lead the campaign against the Rebellion of the Seven Kingdoms. The emperor bestowed upon him a fortune in gold, which Dou Ying placed in the corridor, allowing passing officers to take what they needed for expenses, keeping nothing for himself. After quelling the rebellion, he was enfeoffed as Marquis of Wuqi and later served as Chancellor, amassing such power that countless sycophants flocked to his gate.
During the Rebellion of the Seven Kingdoms, General Guan Fu charged into the rebel camp of the Prince of Wu with only a dozen cavalry, killing and wounding dozens of enemies, earning fame across the land for his ferocity. He was a man of unyielding integrity, who despised flattering the powerful but treated the poor and lowly with genuine respect.
After Empress Dowager Dou's death, Dou Ying gradually lost his influence, and many sycophantic guests who had once flocked to him began to drift away, eager to curry favor with new powers. Only Guan Fu remained as before, treating Dou Ying with deep respect and enjoying a warm rapport with him, often lamenting that they had not become close friends sooner.
Later, Guan Fu, angered by a group of snobs publicly disrespecting Dou Ying, stood up for him and offended the Empress Dowager's brother, the all-powerful Marquis of Weiqi, Tian Fen, and was executed for it.
The idiom "regret not knowing each other earlier" is often written as "regretful for late acquaintance," describing the regret of not having met the other person sooner.
Source: *Records of the Grand Historian*, "Biography of Weiqi and Wu'an Marquises"
Meaning of the Idiom: Later, the Chinese idiom "恨相知晚" came to describe regret of not having met the other person sooner.