'Brown study' definitions:

Definition of 'brown study'

noun

A state of deep absorption or thoughtfulness

From: GCIDE

Brown \Brown\ (broun), a. [Compar. Browner; superl. Brownest.] [OE. brun, broun, AS. br?n; akin to D. bruin, OHG. br?n, Icel. br?nn, Sw. brun, Dan. bruun, G. braun, Lith. brunas, Skr. babhru. [root]93, 253. Cf. Bruin, Beaver, Burnish, Brunette.] Of a dark color, of various shades between black and red or yellow. [1913 Webster]
Cheeks brown as the oak leaves. --Longfellow. [1913 Webster]
Brown Bess, the old regulation flintlock smoothbore musket, with bronzed barrel, formerly used in the British army.
Brown bread (a) Dark colored bread; esp. a kind made of unbolted wheat flour, sometimes called in the United States Graham bread. "He would mouth with a beggar though she smelt brown bread and garlic." --Shak. (b) Dark colored bread made of rye meal and Indian meal, or of wheat and rye or Indian; rye and Indian bread. [U.S.]
Brown coal, wood coal. See Lignite.
Brown hematite or Brown iron ore (Min.), the hydrous iron oxide, limonite, which has a brown streak. See Limonite.
Brown holland. See under Holland.
Brown paper, dark colored paper, esp. coarse wrapping paper, made of unbleached materials.
Brown spar (Min.), a ferruginous variety of dolomite, in part identical with ankerite.
Brown stone. See Brownstone.
Brown stout, a strong kind of porter or malt liquor.
Brown study, a state of mental abstraction or serious reverie. --W. Irving. [1913 Webster]