沐猴而冠 A monkey in a hat
Explanation
沐猴而冠，比喻那些表面上看起来很像正人君子，实际却内心阴险，品质低下，欺骗世人的伪君子。
The idiom "沐猴而冠" describes someone who superficially looks like an upright citizen, but is actually insidious and low in character, deceiving the world.
Origin Story
战国时期，楚国有位大臣叫春申君，他权倾朝野，却贪婪无度，喜欢搜刮民脂民膏，给自己建奢华的府邸，还收纳了大量美妾。春申君虽然表面上衣冠楚楚，但实际上内心却很阴险，他经常利用自己的权势，排挤异己，打击政敌。有一次，春申君宴请宾客，席间他指着一个身穿锦衣玉带，却举止粗俗的官员，对客人说：“这个人啊，就像一只戴着帽子的猴子，看起来像个人，实际上却毫无人性，他不过是靠着我的权势才混到这个位置的。”春申君的这番话，也正是他自己的真实写照。
During the Warring States Period, there was a minister in Chu called Chun Shen Jun. He was powerful and influential, but also greedy and ruthless. He plundered the people, built himself luxurious palaces, and took many beautiful concubines into his harem. Chun Shen Jun outwardly appeared elegant and cultured, but inside he was treacherous and unscrupulous. He used his power to oust dissenters and suppress political opponents. Once, Chun Shen Jun held a banquet for his guests. During the banquet, he pointed to an official who was dressed in silk and gems, but was also rude and crude. He said to his guests, "This person here is like a monkey in a hat. He looks like a human being, but in reality he is heartless and inhumane. He got his position only through my power." Chun Shen Jun's words were at the same time an accurate description of himself.
Usage
这个成语用来讽刺那些外表看似正直，内心却阴险狡诈的人。
This idiom is used to satirize those who appear honest on the surface but are actually insidious and cunning.
Examples
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他表面上装得很正派，实际上却是个沐猴而冠的家伙。
ta biao mian shang zhuang de hen zheng pai, shi ji shang que shi ge mu hou er guan de jia huo.
He pretends to be decent on the surface, but in fact he is a hypocrite.
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这种沐猴而冠的做法，只会让人更加瞧不起你。
zhe zhong mu hou er guan de zuo fa, zhi hui rang ren geng jia qiao bu qi ni.
This kind of hypocrisy will only make people look down on you.