Here's how it breaks down:
* Hard nut: Represents someone who is tough, stubborn, or resistant.
* To crack: Indicates the difficulty of breaking through their resistance.
Examples:
* "The negotiations with the union were a hard nut to crack. They refused to budge on any of their demands."
* "He's a hard nut to crack. I've tried to get him to open up about his past, but he just clammed up."
Essentially, it means someone is unyielding and difficult to influence.