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):

sombre
    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:

44 Moby Thesaurus words for "sombre":
   black, bleak, cheerless, dark, darkling, darksome, depressed,
   depressing, dim, dingy, dismal, doleful, dolorous, dreary, dull,
   dusky, foreboding, funereal, gloomy, grave, grey, grim, grim-faced,
   grim-visaged, joyless, leaden, lowering, lugubrious, melancholic,
   melancholy, morbid, morose, mournful, murky, overcast, sad, sedate,
   serious, shadowy, sober, solemn, staid, subfusc, unhappy