Meaning of the word "tough nut to crack" in English

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

tough nut to crack

US /tʌf nʌt tə kræk/
UK /tʌf nʌt tə kræk/

Idiom

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

Example:
The new boss is a real tough nut to crack.
Solving this mathematical problem is proving to be a tough nut to crack.