The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

brown study
    n 1: a state of deep absorption or thoughtfulness

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "brown study":
   Walter Mitty, absence of mind, absentmindedness, absorption,
   abstractedness, abstraction, bad dream, bemusement,
   castle-building, close study, concentration, contemplativeness,
   daydream, daydreamer, daydreaming, deep thought, depth of thought,
   dream, dreaming, engrossment, fantasy, fantasying,
   fit of abstraction, incubus, melancholy, mooning, moonraking, muse,
   musefulness, musing, muted ecstasy, nightmare, pensiveness,
   pipe dream, pipe-dreaming, preoccupation, profound thought,
   reflectiveness, reverie, speculativeness, stargazing, study,
   thoughtfulness, trance, vision, wistfulness, woolgathering