tough nut to crack
- (idiomatic) A problem that is challenging to solve.
- (idiomatic, by extension) A situation, person, group, etc. which is difficult to deal with.
- (idiomatic) A place, opportunity, etc. to which it is difficult to gain entry.
- An amount that is difficult to finance.
- Tough/er/est nut to crack is by far the most common form in COCA (US), about equal to hard/er/est nut to crack at BNC (UK).