brown study

ベンジャミン・フランクリンの伝記 "Ben and Me" に興味を引く言葉がありました。
Then he would go into a brown study that lasted for hours, occasionally murmuring, "I wonder, I wonder."
"brown study" とは何でしょう？　早速辞書を見ます。
Dictionary.com： deep, serious absorption in thought: Lost in a brown study, she was oblivious to the noise.
"brown study" の語源が気になり調べるとwww.word-detective.comに次ぎの説明がありました。
The key to understanding "brown study" lies in the fact that in the phrase both "brown" and "study" are used in senses now long obsolete (and antiquated even in the 19th century). "Brown study" itself is surprisingly old, dating back to the early 16th century. At that time, "brown" was a synonym for "gloomy or serious," drawing on the symbolism of its darkness much as we speak today of being "in a black mood." The noun "study" in the sense of "state of reverie or contemplation" had first appeared in the 14th century. Together, "brown study" when it first appeared meant "a state of gloomy meditation or thought." Gradually, however, the mood of "brown study" lightened somewhat, and, to the extent it is used today (which isn't often), it now means simply "a state of daydreaming or deep thought."