Liu Ban was a Han Dynasty man whose ancestors had served as kings in the Chu region for generations. By his father's time, Wang Mang—nephew of Emperor Yuan's empress—usurped the Han throne, renamed the dynasty Xin, and demoted Liu Ban's father to commoner status.
Liu Ban lost his father as a child and lived alone with his mother. When Wang Mang was killed by the Lulin rebels, Liu Ban's mother heard that Liu Xuan, a distant imperial relative, had declared himself emperor in Chang'an, so she and her brothers took Liu Ban to seek refuge. Before they reached Chang'an, the Red Eyebrow rebels stormed the city and Liu Xuan was executed, forcing them to wander aimlessly until they finally settled in Gansu. Throughout the journey, despite the hardships, Liu Ban studied diligently every day. His mother and uncle scolded him, "We can't even guarantee our survival—why are you still reading books!" But Liu Ban ignored them and kept studying tirelessly.
After Emperor Guangwu of the Han Dynasty, Liu Xiu, had quelled the various separatist forces that had emerged during Wang Mang's reign, Liu Ban traveled from Gansu to the capital Luoyang, where he inherited his father's title and became the Prince of Chu.
Liu Ban repeatedly offered sound policy suggestions to the imperial court, all of which the emperor adopted. Yangzhou Inspector Guan Xun submitted a memorial praising Liu Ban, stating that he "never spoke without reason" and "acted without causing resentment," and requested that Emperor Ming of Han commend him. The emperor then appointed Liu Ban as Commandant of the Capital, responsible for maintaining order in the capital region.
Later, the idiom "Irreproachable Words" came to describe speech that is always perfectly correct.
Source: *Book of the Later Han*, "Biography of Liu Ban"
Meaning of the Idiom: Later, the Chinese idiom "口无择言" came to describe how speech that is always perfectly correct.