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.