A hard/tough nut (to crack) spoken language noun phrase
To refer to a problem hard to deal with or a person difficult to understand.
Jane's boyfriend is a hard nut and I hardly get across what he says.
To get through his team's defense is a tough nut to crack.
We have doubled security and make this place a hard nut to crack.
Though he is a hard nut, trying to understand students is the mission of a teacher like me. Don't worry.
A problem or difficult circumstance
To have to accept some troubles or worries that must be handled by oneself
That is the biggest or most difficult problem (with the situation being discussed)
Completely controlling over a scenario or situation.
Someone who is totally bad at doing something.
Mary really can't sing for toffee.