Definition of "hard nut to crack" in English

Noun

1. A problem that is challenging to solve.

2. A situation, person, group, etc. which is difficult to overcome or deal with.

3. A place, opportunity, etc. to which it is difficult to gain entry.

4. An amount that is difficult to finance.

5. Used other than figuratively or idiomatically: see hard,‎ nut,‎ crack.