乳臭未干 wet behind the ears
Explanation
形容人年轻、缺乏经验，常用来指责或讽刺对方不够成熟，缺乏阅历。
This idiom describes someone who is young and lacks experience. It is often used to criticize or mock someone for not being mature enough or lacking experience.
Origin Story
话说当年，北宋时期，杨家将战功赫赫，令敌人闻风丧胆。杨家有位年轻的将领杨文广，天生神勇，武功高强，但因为年纪尚小，经验不足，经常被人嘲笑为“乳臭未干”。有一次，辽国入侵，宋朝派杨文广率领军队前去抵抗。杨文广初出茅庐，不知深浅，便率领士兵冲锋陷阵，结果被辽军打得大败而归。回到军营后，老将们都对杨文广的鲁莽行为感到十分不满，纷纷指责他“乳臭未干，不知天高地厚”。杨文广自知理亏，便虚心向老将们请教。老将们见他肯虚心学习，便耐心地给他讲解了战争的策略和战术。杨文广认真听取了老将们的教诲，并努力学习作战经验。经过一段时间的磨砺，杨文广的作战能力有了很大的提高。后来，在与辽军的战斗中，杨文广凭借着自己的勇猛和智慧，带领士兵取得了胜利，一举击溃了辽军。从此，杨文广不再被人嘲笑为“乳臭未干”，而是成为了北宋著名的将领。
It is said that during the time of the Northern Song Dynasty, the Yang family, with their heroes, had won many victories and put their enemies in fear and awe. There was a young general of the Yang family, Yang Wen Guang, who was naturally a hero and had great martial arts skills. But because he was still very young and had no experience, he was often mocked as "still wet behind the ears." Once, when the Liao Dynasty invaded, the Song Dynasty sent Yang Wen Guang with his troops to resist the enemy. Yang Wen Guang, who had just started his military career and didn’t know what he was getting into, led his soldiers into battle, but was defeated by the Liao troops and returned to his camp. When he arrived at the camp, the old generals were very dissatisfied with Yang Wen Guang's reckless actions and accused him of being "a wet-behind-the-ears kid who doesn’t know where the sky is and where the earth is." Yang Wen Guang realized his mistake and humbly asked the old generals for advice. When the old generals saw that he was willing to learn humbly, they patiently taught him about war strategies and tactics. Yang Wen Guang listened attentively to the advice of the old generals and made efforts to learn combat experience. After a period of time, Yang Wen Guang's combat ability greatly improved. Later, in the battle against the Liao troops, Yang Wen Guang led his soldiers to victory with his courage and wisdom, defeating the Liao troops in one fell swoop. From then on, Yang Wen Guang was no longer mocked as "wet behind the ears", but became a famous general of the Northern Song Dynasty.
Usage
这个成语一般用来形容年轻人缺乏经验，不懂事，语气带有轻蔑的意味。
This idiom is generally used to describe young people who lack experience, are not sensible, and the tone is somewhat contemptuous.
Examples
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这个乳臭未干的小孩，竟敢和我顶嘴！
zhe ge ru chou wei gan de xiao hai, jing gan he wo ding zui!
This young kid, still wet behind the ears, dares to talk back to me!
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你才二十岁，乳臭未干，不要以为自己什么都懂！
ni cai er shi sui, ru chou wei gan, bu yao yi wei zi ji shen me dou dong!
You're only twenty years old, still wet behind the ears, don't think you know everything!
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别看他乳臭未干，可是这小子鬼主意多得很！
bie kan ta ru chou wei gan, ke shi zhe xiao zi gui zhu yi duo hen!
Don't underestimate him, even though he's still wet behind the ears, this kid is full of cunning ideas!