A hard nut to crack is a problem or a person that is very difficult to deal with. That contract is a hard nut to crack.
It works for both puzzles and people, and of a person it means unyielding rather than unpleasant. There is admiration in it more often than complaint.
1phrasea hard nut to crackA problem or a person that is very difficult to deal with.
“The old man on the committee was a hard nut to crack.”
a hard nut to crack is a phrase.
Practised in Idioms