露出马脚 reveal one's true colors
Explanation
比喻事情真相败露，隐瞒不住。
The idiom means that the truth or secret is revealed.
Origin Story
从前，有一个名叫张三的狡猾商人，他以低价收购劣质茶叶，然后高价卖给不知情的顾客。为了掩盖茶叶的劣质，他用华丽的包装和夸大的宣传来迷惑顾客。然而，一次偶然的机会，一位经验丰富的茶叶鉴定师发现了张三茶叶的猫腻。鉴定师发现茶叶不仅品质低劣，而且还掺杂了其他杂质。张三的骗局被揭穿，他的奸商面目被世人所知，他试图隐瞒真相，但最终还是露出了马脚。从此以后，张三再也不敢欺骗顾客了，他终于认识到诚实做人的重要性。
Once upon a time, there was a cunning merchant named Zhang San who bought low-quality tea at low prices and sold it at high prices to unsuspecting customers. To cover up the poor quality of the tea, he used beautiful packaging and exaggerated advertising to deceive customers. However, on one occasion, an experienced tea appraiser discovered Zhang San's trickery. The appraiser found that the tea was not only of poor quality but also mixed with other impurities. Zhang San's deception was exposed, and his face as a deceitful merchant was known to the world. He tried to hide the truth, but eventually he revealed his true colors. From then on, Zhang San never dared to deceive customers again, and he finally realized the importance of honesty.
Usage
用于形容事情真相败露，隐瞒不住。
Used to describe when the truth or secret is revealed, it can not be hidden.
Examples
-
他企图隐瞒真相，但最终还是露出了马脚。
ta qitu yanman zhenxiang,dan zui zhong haishi lou chu le ma jiao.
He tried to hide the truth, but eventually his deception was revealed.
-
他的谎言很快就露出了马脚。
ta de huangyan hen kuai jiu lou chu le ma jiao
His lie was quickly exposed.