The Collaborative International Dictionary of English v.0.48:

Scowl \Scowl\ (skoul), v. i. [imp. & p. p. Scowled (skould);
   p. pr. & vb. n. Scowling.] [Akin to Dan. skule; cf. Icel.
   skolla to skulk, LG. schulen to hide one's self, D. schuilen,
   G. schielen to squint, Dan. skele, Sw. skela, AS. sceolh
   squinting. Cf. Skulk.]
   1. To wrinkle the brows, as in frowning or displeasure; to
      put on a frowning look; to look sour, sullen, severe, or
      angry.
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            She scowled and frowned with froward countenance.
                                                  --Spenser.
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   2. Hence, to look gloomy, dark, or threatening; to lower.
      "The scowling heavens." --Thomson.
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