hard nut to crack

1. Also, tough nut to crack. A difficult problem; also, an individual who is difficult to deal with. For example, This assignment is a hard nut to crack, or It won't be easy getting her approval; she's a tough nut to crack. This metaphoric expression alludes to hard-shelled nuts like walnuts. [Early 1700s]

Example Sentences

“It’s a very hard nut to crack. The Russians could spend many months and much effort storming Lysychansk,” military analyst Oleh Zhdanov told the AP.

From Seattle Times

“Bob is reclusive and quiet and a hard nut to crack,” Pullman said.

From New York Times

Voice control is a hard nut to crack.

From The Verge

“This place is a hard nut to crack,” Newgarden said.

From Los Angeles Times

Ukraine has proved a hard nut to crack, and the reaction of Western countries, particularly Germany, has been far fiercer than he thought.

From BBC