The phrase "hard nut" means a person who is tough or difficult to deal with.
Sentences with «hard nut»
The judiciary can have some effect on cost recovery and payments made by defendants but determining how the industry shares its costs is a much harder nut to crack. (newlawjournal.co.uk)
Parental engagement, despite considerable investment, is too hard a nut to crack and there are other, safer bets to improving outcomes. (educationendowmentfoundation.org.uk)