A brown study

Photo by Yaroslav Shuraev on Pexels.com

https://dictionary.cambridge.org/dictionary/english/a-brown-study?topic=thinking-and-contemplating

A brown study

old-fashioned mood in which you are very involved in your own thoughts and not payingattention to anything else:I could see she was in a brown study.He sank back on his pillow and fell into a brown study.