Bartleby provides a handy reference to “brown study” in the classic literature. (Only 4 results? Search at Project Gutenberg on the term and see a lot more–389, with the term in context, to boot. Where are the online concordances of great literature, he cries?)
World Wide Words helpfully provides an etymology.
For reasons known only to myself (and often, not then), I’ve chosen it or forms of it for my online moniker for the last few years.
Brownstudy's Blog 
From an article at World Wide Words:The first example is a surprisingly modern-sounding bit of sage advice in a book called Dice-Play of 1532: “Lack of company will soon lead a man into a brown study”.http://www.worldwidewords.org/qa/qa-bro2.htm
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