Merriam-Webster Collegiate Dictionary - sullen

 adjective  Etymology: Middle English solein solitary, from Anglo-French sulein, solain, perhaps from sol, soul single, sole + -ain after Old French soltain solitary, private, from Late Latin solitaneus, ultimately from Latin solus alone  Date: 14th century  1.  a. gloomily or resentfully silent or repressed a ~ crowd  b. suggesting a ~ state ; lowering a ~ countenance  2. dull or somber in sound or color  3. dismal, gloomy a ~ morning  4. moving sluggishly a ~ river  • ~ly adverb  • ~ness noun Synonyms:  ~, glum, morose, surly, sulky, crabbed, saturnine, gloomy mean showing a forbidding or disagreeable mood. ~ implies a silent ill humor and a refusal to be sociable remained ~ amid the festivities. glum suggests a silent dispiritedness a glum candidate left to ponder a stunning defeat. morose adds to glum an element of bitterness or misanthropy morose job seekers who are inured to rejection. surly implies gruffness and ~ness of speech or manner a typical surly teenager. sulky suggests childish resentment expressed in peevish ~ness grew sulky after every spat. crabbed applies to a forbidding morose harshness of manner the school's notoriously crabbed headmaster. saturnine describes a heavy forbidding aspect or suggests a bitter disposition a saturnine cynic always finding fault. gloomy implies a depression in mood making for seeming ~ness or glumness a gloomy mood ushered in by bad news.