Showing a brooding ill humor or silent resentment; morose or sulky. Gloomy or somber in tone, color, or portent: sullen, gray skies. Sluggish; slow: the sullen current of a canal. Broodingly and sullenly unhappy: dour, gloomy, glum, moody, morose, saturnine, sour, sulky, surly. See happy/unhappy. Characterized by or expressive of a foreboding somberness: dark, lowery. See warn/invite.