Why did courtiers in Qing Dynasty call the word slave slave a derogatory term?

Only the Eight Banners princes and ministers can call themselves slaves. Because after all, the Eight Banners system has a personal attachment with traces of slavery. The emperor is the master of the Eight Banners, and other Eight Banners (including Manchu Eight Banners, Mongolian Eight Banners, Han Army Eight Banners and Ministry of Internal Affairs Three Banners) all consider themselves the slaves of the emperor.

Han Chen does not need to call himself a slave, but a minister.

At that time, a famous minister of Han nationality in Qianlong dynasty learned to call himself a slave in order to show his close relationship with the emperor. If Emperor Qianlong did not accept it, he would not have to call himself a slave and could continue to be a vassal.

When Cen Yu Ying, Governor of Guangdong and Guangxi, met Empress Dowager Cixi in the late Qing Dynasty, the old Cen, who was a military commander, didn't understand. He learned to lead him as a prince of courtesy and called himself a slave, which puzzled Empress Dowager Cixi. After Lao Cen knelt down and went out, Cixi asked people around her to check with the Ministry of Internal Affairs. When did she raise the flag for Cen's family? Later, it spread and became a joke.