What is the meaning of sullen?

adjective

• Showing a brooding ill humor.
  • usage: "a dark scowl"; "the proverbially dour New England Puritan"; "a glum, hopeless shrug"; "he sat in moody silence"; "a morose and unsociable manner"; "a saturnine, almost misanthropic young genius- Bruce Bliven; "a sour temper"; "a sullen crowd"
  • synonyms: dark, dour, glowering, glum, moody, morose, saturnine, sour
• Darkened by clouds.
  • usage: "a heavy sky"
  • synonyms: heavy, lowering, threatening