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.
      [1913 Webster]

            She scowled and frowned with froward countenance.
                                                  --Spenser.
      [1913 Webster]

   2. Hence, to look gloomy, dark, or threatening; to lower.
      "The scowling heavens." --Thomson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scowling
    adj 1: sullen or unfriendly in appearance [syn: beetle-browed,
           scowling]