The Collaborative International Dictionary of English v.0.48:

Mouth \Mouth\ (mouth), n.; pl. Mouths (mou[th]z). [OE. mouth,
   mu[thorn], AS. m[=u][eth]; akin to D. mond, OS. m[=u][eth],
   G. mund, Icel. mu[eth]r, munnr, Sw. mun, Dan. mund, Goth.
   mun[thorn]s, and possibly L. mentum chin; or cf. D. muil
   mouth, muzzle, G. maul, OHG. m[=u]la, Icel. m[=u]li, and Skr.
   mukha mouth.]
   1. The opening through which an animal receives food; the
      aperture between the jaws or between the lips; also, the
      cavity, containing the tongue and teeth, between the lips
      and the pharynx; the buccal cavity.
      [1913 Webster]

   2. Hence: An opening affording entrance or exit; orifice;
      aperture; as:
      (a) The opening of a vessel by which it is filled or
          emptied, charged or discharged; as, the mouth of a jar
          or pitcher; the mouth of the lacteal vessels, etc.
      (b) The opening or entrance of any cavity, as a cave, pit,
          well, or den.
      (c) The opening of a piece of ordnance, through which it
          is discharged.
      (d) The opening through which the waters of a river or any
          stream are discharged.
      (e) The entrance into a harbor.
          [1913 Webster]

   3. (Saddlery) The crosspiece of a bridle bit, which enters
      the mouth of an animal.
      [1913 Webster]

   4. A principal speaker; one who utters the common opinion; a
      mouthpiece.
      [1913 Webster]

            Every coffeehouse has some particular statesman
            belonging to it, who is the mouth of the street
            where he lives.                       --Addison.
      [1913 Webster]

   5. Cry; voice. [Obs.] --Dryden.
      [1913 Webster]

   6. Speech; language; testimony.
      [1913 Webster]

            That in the mouth of two or three witnesses every
            word may be established.              --Matt. xviii.
                                                  16.
      [1913 Webster]

   7. A wry face; a grimace; a mow.
      [1913 Webster]

            Counterfeit sad looks,
            Make mouths upon me when I turn my back. --Shak.
      [1913 Webster]

   Down at the mouth or Down in the mouth, chapfallen; of
      dejected countenance; depressed; discouraged. [Obs. or
      Colloq.]

   Mouth friend, one who professes friendship insincerely.
      --Shak.

   Mouth glass, a small mirror for inspecting the mouth or
      teeth.

   Mouth honor, honor given in words, but not felt. --Shak.

   Mouth organ. (Mus.)
      (a) Pan's pipes. See Pandean.
      (b) An harmonicon.

   Mouth pipe, an organ pipe with a lip or plate to cut the
      escaping air and make a sound.

   To stop the mouth, to silence or be silent; to put to
      shame; to confound.

   To put one's foot in one's mouth, to say something which
      causes one embarrassment.

   To run off at the mouth, to speak excessively.

   To talk out of both sides of one's mouth, to say things
      which are contradictory.
      [1913 Webster +PJC]

            The mouth of them that speak lies shall be stopped.
                                                  --Ps. lxiii.
                                                  11.
      [1913 Webster]

            Whose mouths must be stopped.         --Titus i. 11.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Down \Down\, a.
   1. Downcast; as, a down look. [R.]
      [1913 Webster]

   2. Downright; absolute; positive; as, a down denial. [Obs.]
      --Beau. & Fl.
      [1913 Webster]

   3. Downward; going down; sloping; as, a down stroke; a down
      grade; a down train on a railway.
      [1913 Webster]

   Down draught, a downward draft, as in a flue, chimney,
      shaft of a mine, etc.

   Down in the mouth, Down at the mouth chopfallen;
      dejected.
      [1913 Webster]

WordNet (r) 3.0 (2006):

down in the mouth
    adj 1: filled with melancholy and despondency ; "gloomy at the
           thought of what he had to face"; "gloomy predictions"; "a
           gloomy silence"; "took a grim view of the economy"; "the
           darkening mood"; "lonely and blue in a strange city";
           "depressed by the loss of his job"; "a dispirited and
           resigned expression on her face"; "downcast after his
           defeat"; "feeling discouraged and downhearted" [syn:
           gloomy, grim, blue, depressed, dispirited,
           down(p), downcast, downhearted, down in the
           mouth, low, low-spirited]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "down in the mouth":
   bowed-down, cast down, dashed, dejected, depressed, despairing,
   despondent, desponding, discouraged, disheartened, dispirited,
   down, downcast, downhearted, drooping, droopy, feeling low,
   heartless, hypochondriac, hypochondriacal, in low spirits,
   in the depths, in the doldrums, in the dumps, languishing, low,
   low-spirited, pessimistic, pining, spiritless, subdued, suicidal,
   weary of life, woebegone, world-weary