in a brown study

Most people spend between 30 and 47 percent of their waking hours spacing out, drifting off, lost in thought, woolgathering, in a brown study or building castles in the air.

When confined in his own house and quite poor and had no money to waste on follies, he caused the little room in which he read and wrote to be painted brown, that he might say to every visitor, "You see, I am in a brown study."

Probably he was in a brown study about something.

And one by one the little troop turned and trailed despondently from the house, Clyne with his chin sunk on his breast, Bishop in a brown study, the other men staring blankly before them.

And he was deep in a brown study, in which love had no part, when an exclamation, at once of surprise and admiration, recalled him to the present.