A hard/tough nut - Idioms by The Free Dictionary
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.
a tough (or hard) nut (to crack) someone who is difficult to deal with or hard to beat; a formidable person. informal
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.