The Abbé listened attentively, and seemed to be in a brown study. He soon took his leave of the two foreigners after a most tender embrace.
Voltaire. Candide
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“It seems to me”, said she, “that you are in some brown study, what colours you might best wear for your lady.”
John Lyly. Euphues: The Anatomy of Wit (1578)
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Linton sank back on his pillow and fell into a brown study.
Emily Brontë. Wuthering Heights (1847)
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So gathering up the shavings with another grin, and throwing them into the great stove in the middle of the room, he went about his business, and left me in a brown study.