What is the origin of the idiom "brown study" and when to use it?

The idiom "brown study" has its origins in Middle English, where the term "brown" was used to describe something that was gloomy, dull, or obscure. In this context, a "brown study" referred to a state of deep contemplation or daydreaming, often characterized by someone being lost in their thoughts. It is used to describe a state of deep, often absentminded contemplation or daydreaming in which a person becomes absorbed in their thoughts.