The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. To inssinuate contempt by a covert expression; to speak
      derisively.
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            I could be content to be a little sneared at.
                                                  --Pope.
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   3. To show mirth awkwardly. [R.] --Tatler.
      [1913 Webster]

   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.
          [1913 Webster]

                And sneers as learnedly as they,
                Like females o'er their morning tea. --Swift.
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                Midas, exposed to all their jeers,
                Had lost his art, and kept his ears. --Swift.
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                The fop, with learning at defiance,
                Scoffs at the pedant and science. --Gay.
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The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            "A ship of fools," he sneered.        --Tennyson.
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   2. To treat with sneers; to affect or move by sneers.
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            Nor sneered nor bribed from virtue into shame.
                                                  --Savage.
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The Collaborative International Dictionary of English v.0.48:

Sneer \Sneer\, n.
   1. The act of sneering.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sneer
    n 1: a facial expression of contempt or scorn; the upper lip
         curls [syn: sneer, 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"

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "sneer":
   barrack, be above, be contemptuous of, care nothing for, chaff,
   contemn, contempt, cut, cut at, cut direct, deride, derision,
   despise, dig at, disdain, disparage, disprize, disregard, dump on,
   feel contempt for, feel superior to, fleer, flout, gibe, gird,
   grin, hold beneath one, hold cheap, hold in contempt, humiliation,
   insult, jab, jab at, jape, jeer, jeer at, jeering, jest, knock,
   laugh at, leer, look down upon, misprize, mock, mockery, pooh,
   pooh-pooh, put down, quip, rail at, rally, rank low, rank out,
   rebuff, repulse, revile, ridicule, sardonic grin, scoff, scorn,
   scornful laugh, scout, set at naught, slam, slap at, slight, smile,
   smirk, sneer at, sneering, sneeze at, snicker, sniff, sniff at,
   snigger, snort, snort at, snub, spurn, spurning, swipe, taunt,
   the cold shoulder, the go-by, think nothing of, twit, underrate