Meaning of the word "a hard nut to crack" in English

What does "a hard nut to crack" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

a hard nut to crack

US /ə hɑrd nʌt tu kræk/
UK /ə hɑːd nʌt tu kræk/
"a hard nut to crack" picture

Idiom

a problem that is difficult to solve or a person who is difficult to understand or deal with

Example:
This math problem is really a hard nut to crack.
My boss is a hard nut to crack; he rarely changes his mind.