A tough customer/cookie noun phrase informal
A person whose character is strong and determined, and is not easily influenced by other people
I believe that there will be no obstacles that can stop him from succeeding because he is a tough cookie.
No one can make her change her mind because she is a tough customer.
Small and steady efforts can bring about big results.
State something in a very strong, clear and effective way
Someone who has a strong personality
Persistence and determination will achieve success.
To do whatever it takes to achieve something, whether ending in success or failure
The origin of this noun phrase is not clear.