Source : Webster's Revised Unabridged Dictionary (1913)

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.

            She scowled and frowned with froward countenance.
                                                  --Spenser.

   2. Hence, to look gloomy, dark, or threatening; to lower.
      ``The scowling heavens.'' --Thomson.

Source : WordNet®

scowling
     adj : sullen or unfriendly in appearance [syn: {beetle-browed}]