1
general::
Daydreaming or deeply contemplative, as in Margaret sits in the library, in a brown study. This term dates from the late 1500s, and although by then in a study had long meant "lost in thought," the reason for adding brown is unclear. Moreover, the present idiom Also is ambiguous, some holding that it denotes genuine thoughtfulness and others that it signifies absentmindedness.
American Heritage Idioms