a hard/tough nut

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

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

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