The idiom means that ambition is very obvious and well known.
Si Mazhao was Chen Wei in the Three Kingdoms period. After his stepbrother, he continued to develop the power of Sima family, and his ambition to usurp the throne was well known.
Si Mazhao's father is Sima Yi, a general of Wei. After the death of Cao Rui, Wei Mingdi, Cao Shuang and Sima Yi were entrusted to assist Cao Binfang of Qi State to govern the world. Cao Shuang and Sima Yi pushed each other out. After a fierce power struggle, Sima Yi tried to punish Cao Shuang, and the military and political power of Wei fell into Sima Yi's hands.
After Sima Yi's death, his eldest son, Sima Shi, quickly abolished Cao Fang, who was an adult but failed to lead the government, and made 13-year-old Cao Mao emperor, more powerful than Sima Yi, but soon died of illness. When Sima Shi was seriously ill, he gave power to his younger brother Si Mazhao.
After Si Mazhao came to power, he was even more ambitious and always wanted to replace Cao Mao. He kept rooting out dissidents and attacking political enemies. Young Cao Mao knew that even if he was a "puppet" emperor, he wouldn't last long, and he would be removed by Si Mazhao sooner or later, so he planned to take risks and kill Si Mazhao by surprise.
One day, Cao Pi called his confidant ministers and said, "Everyone knows Si Mazhao's heart. I can't stand the shame of being overthrown in vain. I want you to crusade against him with me. " Several ministers knew that doing so was tantamount to throwing a moth into a fire and advised him to be patient for a while. A man named Wang Jing who was present at that time said to Cao Mao: "Now the power is in Si Mazhao's hands, and the civil servants in the Qing Dynasty are all his people; Your majesty, you are weak and weak, and the consequences are unimaginable if you act rashly. You should think carefully. "
Cao Mao refused to accept the suggestion and personally led hundreds of servants and guards to attack Si Mazhao. Who knows that some ministers have reported the news to Si Mazhao. Si Mazhao immediately sent troops to intercept and killed Cao Mao.
Later, people used the phrase "Si Mazhao's heart is known to all" to illustrate that the conspirators' ambition is very obvious and has been known.