Refers to something utterly impossible or highly improbable to happen; an unattainable feat.
想让他在一天内完成这个项目，简直是乌头马角，根本不可能。
Expecting him to complete this project in a single day is simply an impossible feat, absolutely out of the question.
Era:
Ancient
Frequency:
Synonym:
Story:
During the late Warring States period, Prince Dan of Yan was held as a hostage in Qin. After a conflict with King Ying Zheng of Qin, he was imprisoned.
Prince Dan pleaded to be allowed to return to his home country. King Qin replied, "I will grant your request only when a crow's head turns white and a horse grows horns."
Prince Dan could only sigh to the sky in despair. Perhaps heaven had eyes; a white-headed crow flew by outside his prison cell.
Faced with this extraordinary sight, the King of Qin had no choice but to send Prince Dan back to Yan.
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