The Collaborative International Dictionary of English v.0.48:

Frown \Frown\ (froun), v. i. [imp. & p. p. Frowned (fround);
   p. pr. & vb. n. Frowning.] [OF. froignier, F. frogner, in
   se refrogner, se renfrogner, to knit the brow, to frown;
   perh. of Teutonic origin; cf. It. in frigno wrinkled,
   frowning, Prov. It. frignare to cringe the face, to make a
   wry face, dial. Sw. fryna to make a wry face,]
   1. To contract the brow in displeasure, severity, or
      sternness; to scowl; to put on a stern, grim, or surly
      look.
      [1913 Webster]

            The frowning wrinkle of her brow.     --Shak.
      [1913 Webster]

   2. To manifest displeasure or disapprobation; to look with
      disfavor or threateningly; to lower; as, polite society
      frowns upon rudeness.
      [1913 Webster]

            The sky doth frown and lower upon our army. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

frowning
    adj 1: showing displeasure or anger

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "frowning":
   beetle-browed, black, black-browed, dark, decorous, dejected,
   demure, dour, dumpish, earnest, formal, glowering, glum, grave,
   grim, grim-faced, grim-visaged, grum, long-faced, lowering,
   melancholy, moodish, moody, mopey, moping, mopish, morose, mumpish,
   scowling, sedate, serious, sober, sober-minded, sobersided, solemn,
   somber, staid, stone-faced, straight-faced, sulky, sullen, surly,
   thoughtful, unsmiling, weighty