The Collaborative International Dictionary of English v.0.48:

Somber \Som"ber\, Sombre \Som"bre\(?; 277), a. [F. sombre; cf.
   Sp. sombra, shade, prob. from LL. subumbrare to put in the
   shade; L. sub under + umbra shade. See Umbrage.]
   1. Dull; dusky; somewhat dark; gloomy; as, a somber forest; a
      somber house.
      [1913 Webster]

   2. Melancholy; sad; grave; depressing; as, a somber person;
      somber reflections.
      [1913 Webster]

            The dinner was silent and somber; happily it was
            also short.                           --Beaconsfield.
      [1913 Webster] Somber

The Collaborative International Dictionary of English v.0.48:

Somber \Som"ber\, Sombre \Som"bre\, v. t.
   To make somber, or dark; to make shady. [R.]
   [1913 Webster] Somber

The Collaborative International Dictionary of English v.0.48:

Somber \Som"ber\, Sombre \Som"bre\, n.
   Gloom; obscurity; duskiness; somberness. [Obs.]
   [1913 Webster] Somberly

WordNet (r) 3.0 (2006):

somber
    adj 1: lacking brightness or color; dull; "drab faded curtains";
           "sober Puritan grey"; "children in somber brown clothes"
           [syn: drab, sober, somber, sombre]
    2: grave or even gloomy in character; "solemn and mournful
       music"; "a suit of somber black"; "a somber mood" [syn:
       somber, sombre, melancholy]

Moby Thesaurus II by Grady Ward, 1.0:

190 Moby Thesaurus words for "somber":
   Acheronian, Acherontic, Cimmerian, Quaker-colored, Stygian, acier,
   apocalyptic, ashen, ashy, bad, baleful, baneful, black, blackish,
   bleak, bodeful, boding, caliginous, canescent, cinereous, cinerous,
   clouded, cloudy, creamy, dapple, dapple-gray, dappled,
   dappled-gray, dark, dark and gloomy, dark-colored, darkish,
   darksome, dead, deadened, decorous, delicate, demure, depressing,
   depressive, dim, dingy, dire, dismal, dispiriting, doomful,
   dove-colored, dove-gray, drab, drear, drearisome, dreary, dull,
   dusk, dusky, dusty, earnest, eggshell, evil, evil-starred, fateful,
   flat, foreboding, formal, frowning, funebrial, funereal,
   glaucescent, glaucous, gloomful, glooming, gloomy, gloss, grave,
   gray, gray-black, gray-brown, gray-colored, gray-drab, gray-green,
   gray-spotted, gray-toned, gray-white, grayed, grayish, grim,
   grim-faced, grim-visaged, griseous, grizzle, grizzled, grizzly,
   ill, ill-boding, ill-fated, ill-lighted, ill-lit, ill-omened,
   ill-starred, inauspicious, iridescent, iron-gray, lackluster,
   lead-gray, leaden, lifeless, light, livid, long-faced, lowering,
   lusterless, mat, mellow, menacing, mother-of-pearl, mouse-colored,
   mouse-gray, mousy, murky, nacreous, nigrescent, no-nonsense,
   obscure, of evil portent, ominous, opalescent, overcast, pale,
   pastel, patinaed, pearl, pearl-gray, pearly, portending,
   portentous, quiet, sad, saturnine, sedate, semigloss, serious,
   silver, silver-gray, silvered, silvery, simple, sinister,
   slate-colored, slaty, smoke-gray, smoky, sober, sober-minded,
   sobersided, soft, soft-colored, soft-hued, softened, solemn,
   sombrous, staid, steel-gray, steely, stone-colored, stone-faced,
   stormy, straight-faced, subdued, subtle, swart, swarthy, sweet,
   taupe, tender, tenebrous, thoughtful, threatening, triste,
   unfavorable, unfortunate, unlucky, unpromising, unpropitious,
   unsmiling, untoward, wan, weariful, wearisome, weary, weighty