During the Chu-Han contention, Liu Bang and Xiang Yu were locked in a stalemate at Xingyang. Because the King of Western Wei, Wei Bao, was about to betray him, Liu Bang sent Li Yiji to dissuade him. Wei Bao ignored Li Yiji's advice and said to him, "Liu Bang is arrogant and rude to his subordinates, treating his ministers like slaves. I am unwilling to cooperate with him any longer."
Emperor Liu Bang, furious with Wei Bao, decided to launch a campaign against him. Before setting out, he asked his advisor Li Yiji, "Who is Wei's chief general?" "Bo Zhi," Li replied. Liu Bang smirked, "That man still smells of mother's milk—he's no match for my Han Xin. Who commands Wei's cavalry?" "Feng Jing." "Though virtuous, he cannot rival my Guan Ying. And the infantry commander?" "Xiang Tuo." Liu Bang beamed, "He is no equal to my Cao Can. With this lineup, I have nothing to fear in conquering Wei Bao."
Emperor Liu Bang then dispatched Han Xin, Guan Ying, and Cao Shen to attack Wei Bao. Wei Bao was defeated and captured, but Liu Bang spared his life, ordering him to garrison Xingyang instead.
Soon, Xiang Yu besieged the Han army at Xingyang. General Zhou Ke of Liu Bang said, "Wei Bao has betrayed the Han king twice to side with Xiang Yu. Now Xingyang is in crisis; this man is untrustworthy." So he had him killed. Later, the idiom "still smelling of milk" came to be used to belittle young people.
Source: *Book of Han*, Chapter "Annals of Emperor Gao"
Meaning of the Idiom: Later, the Chinese idiom "口尚乳臭" came to describe how someone is still smelling of milk to belittle young people.