弹冠相庆 to congratulate each other (in the officialdom)
Explanation
比喻官场中一人当了官或升了官，同伙就互相庆贺将有官可做。也用来形容坏人得意的样子。
It is used to describe the situation where officials congratulate each other when one of them is promoted or gets an office. It is also used to describe the smug look of villains.
Origin Story
汉宣帝时，琅琊人王吉和贡禹是很好的朋友，贡禹多次被免职，王吉在官场也很不得志。汉元帝时，王吉被召去当谏议大夫，贡禹听到这个消息很高兴，就把自己的官帽取出，弹去灰尘，准备戴用。果然没多久贡禹也被任命为谏议大夫。这便是成语“弹冠相庆”的由来，它比喻官场中一人得志，同伙就互相庆贺将有官可做。当然，它也常常用来讽刺那些互相勾结、为非作歹的人。
During the reign of Emperor Xuan of Han, Wang Ji and Gong Yu from Langya were good friends. Gong Yu was dismissed from his post several times, and Wang Ji also did not have much success in his official career. During the reign of Emperor Yuan of Han, Wang Ji was summoned to serve as Jianyi Daifu. Gong Yu was very happy to hear this news, so he took out his official hat, brushed off the dust, and prepared to put it on. Sure enough, not long after, Gong Yu was also appointed as Jianyi Daifu. This is the origin of the idiom "弹冠相庆", which describes the situation where one official succeeds, and his colleagues congratulate each other because they will soon also get an office. Of course, it is also often used to satirize those who collude with each other and do evil.
Usage
用于形容官场中人互相庆贺升迁或即将升迁的情景，也常用来讽刺那些互相勾结、为非作歹的人。
It is used to describe the situation where officials congratulate each other on promotions or upcoming promotions, and it is often used to satirize those who collude with each other and do evil.
Examples
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朝中有人，弹冠相庆
zhao zhong you ren, tan guan xiang qing
Those who are in power celebrate each other's promotion
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他升官了，同僚们都弹冠相庆
ta sheng guan le, tongliao men dou tan guan xiang qing
His promotion was celebrated with joy by his colleagues