What is the origin of the idiom "fine kettle of fish" and when to use it?

The idiom "fine kettle of fish" is used to describe a complicated, troublesome, or confusing situation. It originated in Scotland in the late 18th century and has its roots in fishing. In those times, a kettle was used to cook fish, and if the kettle was in a poor state or the fish was spoiled, it would result in a disappointing and messy situation.