a hard nut (to crack)

a hard nut (to crack)

A person, thing, situation, or problem that is particularly difficult to understand, solve, or deal with. I've been dating Jenny for over a year, and I still think she's a hard nut to crack! Figuring out the best way to modernize our product without alienating existing customers is definitely a hard nut to crack. The spy we captured is a hard nut; he hasn't said a thing since we began the interrogation.
See also: hard, nut
Farlex Dictionary of Idioms.

(a) hard nut to crack

 and (a) tough nut to crack
Fig. difficult person or problem to deal with. This problem is getting me down. It's a hard nut to crack. Tom sure is a hard nut to crack. I can't figure him out.
See also: crack, hard, nut
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

hard nut to crack

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]
See also: crack, hard, nut
The American Heritage® Dictionary of Idioms by Christine Ammer.

a hard/tough ˈnut (to ˈcrack)

(informal) a very difficult problem to solve; a very difficult person to deal with: Persuading drivers to leave their cars at home and use public transport will be a very tough nut to crack. You’ll find it difficult to make him change his mind. He’s a tough nut.
See also: hard, nut, tough
Farlex Partner Idioms Dictionary
See also:
  • a hard egg to crack
  • a hard/tough nut
  • a tough nut
  • hard nut to crack
  • bit of rough
  • a bit of rough
  • a mother hen
  • in years
  • make a note of
  • make a note of (something)