This
database is a comprehensive collection of all the American idioms and slang available.
American Idioms are many and varied. We hope you enjoy our collection. We are adding more all the time.
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If something is a tough nut to crack, it is difficult to find the answer or solution. When used about a person, it means that it is difficult to get them to do or allow what you want.
'Hard nut to crack' is an alternative.
Moral fibre is the inner strength to do what you believe to be right in difficult situations
Example: He lacked the moral fibre to be leader
(In American English the correct spelling is 'fiber'.)