Definition
a problem that is very difficult to solve or a person who is very difficult to understand
难以解决的问题或难以理解的人。
This math problem is a real hard nut to crack; I’ve been working on it for hours without any progress.
这道数学题真是块难啃的骨头，我已经花了几个小时还是没有任何进展。
Trying to convince him to change his mind is a tough nut to crack, he is a very difficult person to understand.
试图说服他改变主意是个难题，他是一个非常难以理解的人。
The technical issues of this project are a hard nut to crack, we need to find an innovative solution.
这个项目的技术问题是个难题，我们需要找到一个创新的解决方案。