What is the origin of the idiom "hard nut" and when to use it?

The idiom "hard nut" is used to describe a person who is difficult to deal with or a problem that is challenging to solve. It originated in the late 19th century and draws its metaphorical meaning from the idea of cracking open a nut. Just as some nuts have tough shells that are hard to break, this expression refers to someone who is resistant, unyielding, or stubborn.