What is "hard nut to crack"

alt. 1 (&lit hard nut crack English) 2 (context idiomatic English) A problem that is challenging to solve. 3 (context idiomatic by extension English) A situation, person, group, etc. which is difficult to overcome or deal with. 4 (context idiomatic English) A place, opportunity, etc. to which it is difficult to gain entry. 5 (context idiomatic English) An amount that is difficult to finance. n. 1 (&lit hard nut crack English) 2 (context idiomatic English) A problem that is challenging to solve. 3 (context idiomatic by extension English) A situation, person, group, etc. which is difficult to overcome or deal with. 4 (context idiomatic English) A place, opportunity, etc. to which it is difficult to gain entry. 5 (context idiomatic English) An amount that is difficult to finance.