Noun
(idiomatic) A problem that is challenging to solve.
(idiomatic) A place, opportunity, etc. to which it is difficult to gain entry.
Used other than figuratively or idiomatically: see hard, nut, crack.
(idiomatic, by extension) A situation, person, group, etc. which is difficult to overcome or deal with.
(idiomatic) An amount that is difficult to finance.
Source: en.wiktionary.orgI think America is a hard nut to crack. But once you get a toehold, it's a great place for an entrepreneur because people are so enthusiastic, and you have the most enthusiastic audiences in world. Simon Cowell
On what looks like being another stellar afternoon for O’Brien, Cliffs Of Moher should prove a hard nut to crack in the Tattersalls Gold Cup. Source: Internet
What with an 89 year-old president, deemed something of a ‘pariah’ that has constantly proved a hard nut to crack and refused to let go of the country’s newfound diamond resources? Source: Internet