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]

The Collaborative International Dictionary of English v.0.48:

Frown \Frown\, v. t.
   To repress or repel by expressing displeasure or disapproval;
   to rebuke with a look; as, frown the impudent fellow into
   silence.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Frown \Frown\, n.
   1. A wrinkling of the face in displeasure, rebuke, etc.; a
      sour, severe, or stern look; a scowl.
      [1913 Webster]

            His front yet threatens, and his frowns command.
                                                  --Prior.
      [1913 Webster]

            Her very frowns are fairer far
            Than smiles of other maidens are.     --H.
                                                  Coleridge.
      [1913 Webster]

   2. Any expression of displeasure; as, the frowns of
      Providence; the frowns of Fortune.
      [1913 Webster]

WordNet (r) 3.0 (2006):

frown
    n 1: a facial expression of dislike or displeasure [syn:
         frown, scowl]
    v 1: look angry or sullen, wrinkle one's forehead, as if to
         signal disapproval [syn: frown, glower, lour,
         lower]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "frown":
   angry look, ban, black look, blackball, categorically reject,
   color, deprecate, dirty look, disallow, disapprove, disapprove of,
   discommend, discountenance, disesteem, disfavor, dissent from,
   exclude, flush, frown at, frown down, frown upon, glare, gloom,
   glower, gnarl, grimace, grimace at, growl, knit the brow,
   long face, look askance at, look black, look black upon,
   look daggers, look down upon, look sullen, lour, lower, make a lip,
   make a moue, mantle, moue, mow, nasty look, not approve,
   not go for, not hear of, not hold with, object, object to, offense,
   oppose, ostracize, pique, pout, protest, redden, reject,
   reproving look, say no to, scowl, snap, snarl, spit, sulk,
   take exception to, think ill of, think little of, thumb down,
   umbrage, view with disfavor, wry face