What is tough cookie to crack?

The phrase "tough cookie to crack" is an idiomatic expression that means an individual or situation that is difficult to solve or deal with. It can refer to a person who is stubborn, unyielding, or resistant to change, and it can also be used to describe a problem or challenge that is complex and requires a lot of effort to solve. The origins of this phrase are unclear, but it is believed to have originated in the early 20th century, and it has since become a common expression in the English language.