tough nut to crack

• tougher nut to crack, toughest nut to crack, difficult nut to crack, hard nut to crack

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tough nut to crack (plural tough nuts to crack)

1. (idiomatic) A problem that is challenging to solve.
2. (idiomatic, by extension) A situation, individual, group, etc. that is difficult to deal with.
3. (idiomatic) A place, opportunity, etc. to which it is difficult to gain entry.
4. (idiomatic) 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).