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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The Collaborative International Dictionary of English v.0.48:

Scowl \Scowl\, v. t.
   1. To look at or repel with a scowl or a frown. --Milton.
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   2. To express by a scowl; as, to scowl defiance.
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The Collaborative International Dictionary of English v.0.48:

Scowl \Scowl\, n.
   1. The wrinkling of the brows or face in frowing; the
      expression of displeasure, sullenness, or discontent in
      the countenance; an angry frown.
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            With solemn phiz, and critic scowl.   --Lloyd.
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   2. Hence, gloom; dark or threatening aspect. --Burns.
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            A ruddy storm, whose scowl
            Made heaven's radiant face look foul. --Crashaw.
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WordNet (r) 3.0 (2006):

scowl
    n 1: a facial expression of dislike or displeasure [syn:
         frown, scowl]
    v 1: frown with displeasure

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "scowl":
   angry look, black look, color, dirty look, flush, frown, glare,
   gloom, glower, gnarl, grimace, growl, knit the brow, long face,
   look black, look daggers, look sullen, lour, lower, make a lip,
   make a moue, mantle, moue, mow, nasty look, offense, pique, pout,
   pull a face, redden, reproving look, snap, snarl, spit, umbrage,
   wry face