Source : Webster's Revised Unabridged Dictionary (1913)

Sneer \Sneer\, n.
   1. The act of sneering.

   2. A smile, grin, or contortion of the face, indicative of
      contempt; an indirect expression or insinuation of
      contempt. ''Who can refute a sneer?'' --Raley.

Sneer \Sneer\, v. i. [imp. & p. p. {Sneered}; p. pr. & vb. n.
   {Sneering}.] [OE. sneren, Dan. sn?rre to snarl or grin (like
   a dog); cf. Prov. E. sneer to grin, sner to snort, snert to
   sneer at. See {Snore}, v. i.]
   1. To show contempt by turning up the nose, or by a
      particular facial expression.

   2. To inssinuate contempt by a covert expression; to speak
      derisively.

            I could be content to be a little sneared at.
                                                  --Pope.

   3. To show mirth awkwardly. [R.] --Tatler.

   Syn: To scoff; gibe; jeer.

   Usage: {Sneer}, {Scoff}, {Jeer}. The verb to sneer implies to
          cast contempt indirectly or by covert expressions. To
          jeer is stronger, and denotes the use of several
          sarcastic reflections. To scoff is stronger still,
          implying the use of insolent mockery and derision.

                And sneers as learnedly as they, Like females
                o'er their morning tea.           --Swift.

                Midas, exposed to all their jeers, Had lost his
                art, and kept his ears.           --Swift.

                The fop, with learning at defiance, Scoffs at
                the pedant and science.           --Gay.

Sneer \Sneer\, v. t.
   1. To utter with a grimace or contemptuous expression; to
      utter with a sneer; to say sneeringly; as, to sneer
      fulsome lies at a person. --Congreve.

            ``A ship of fools,'' he sneered.      --Tennyson.

   2. To treat with sneers; to affect or move by sneers.

            Nor sneered nor bribed from virtue into shame.
                                                  --Savage.

Source : WordNet®

sneer
     n 1: a facial expression of contempt or scorn; the upper lip
          curls [syn: {leer}]
     2: a contemptuous or scornful remark
     v 1: express through a scornful smile; "she sneered her contempt"
     2: smile contemptuously; "she sneered at her little sister's
        efforts to play the song on the piano"