What does the idiom 'a hard nut to crack' signify?

Explanation

The phrase 'a hard nut to crack' refers to a problem that is difficult to solve or a person who is tough to understand or persuade. For instance, you might say, 'This task is a hard nut to crack,' or 'Convincing him is a hard nut to crack.' The expression derives from the idea of a nut with a tough shell, like a walnut, which is not easy to open.