a tough customer/cookie

tough cookie

A strong, determined person who is not easily intimidated, discouraged, or defeated. Our principal was a little lady, but she was one tough cookie! She had a difficult childhood, but she's a tough cookie. I know she'll be a success.
See also: cookie, tough

tough customer

A strong, determined person who is not easily intimidated, discouraged, or defeated. Our principal was a rather petite lady, but she was one tough customer! She had a difficult childhood, but it made her into a tough customer later in life. I wouldn't mess with that guy, he's a pretty tough customer.
See also: customer, tough
Farlex Dictionary of Idioms.

tough cookie

a person who is difficult to deal with. He's a tough cookie, but I can handle him. There was a tough cookie in here this morning who demanded to see the manager.
See also: cookie, tough

tough customer someone

who is difficult to deal with. Some of those bikers are really tough customers. Walt is a tough customer. Just keep away from him.
See also: customer, tough
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

a tough cookie

or

one tough cookie

If someone is a tough cookie or one tough cookie, they are confident, brave and determined. A pushover is not how I'd describe her. Behind that sweet smile, there lies a tough cookie. He has a reputation as one tough cookie.
See also: cookie, tough
Collins COBUILD Idioms Dictionary, 3rd ed.

a tough ˈcustomer/ˈcookie

(informal) a person who knows what they want and is not easily influenced by other people: Self-confident, ambitious and positive, Paula is a tough cookie who is bound to do well.
See also: cookie, customer, tough
Farlex Partner Idioms Dictionary

tough cookie

n. a tough person. There was a tough cookie in here this morning who demanded to see the manager.
See also: cookie, tough

tough customer

n. someone who is difficult to deal with. Bruno is a tough customer. Just keep away from him.
See also: customer, tough
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • a tough cookie
  • be in low spirits
  • appear to
  • a change of heart
  • a mystery to (one)
  • a shoulder to cry on
  • a yellow streak
  • a turn of phrase
  • able to do
  • able to do it