a tough nut

Related to a tough nut: tough nut to crack

a tough nut

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 tough nut to crack! Figuring out the best way to modernize our product without alienating existing customers is definitely a tough nut to crack. The spy we captured is a tough nut; he hasn't said a thing since we began the interrogation.
See also: nut, tough
Farlex Dictionary of Idioms.

a tough nut

or

a hard nut

INFORMAL
If you call someone a tough nut or a hard nut, you mean they have a strong character and are very determined. That Bridget would sort anyone out — she's a tough nut.
See also: nut, tough
Collins COBUILD Idioms Dictionary, 3rd ed.

a tough (or hard) nut (to crack)

someone who is difficult to deal with or hard to beat; a formidable person. informal
See also: nut, tough
Farlex Partner Idioms Dictionary

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 nut (to crack)
  • a hard/tough nut
  • hard nut to crack
  • a bit of rough
  • bit of rough
  • a mother hen
  • in years
  • make a note of
  • make a note of (something)