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.
      [1913 Webster]

            I could be content to be a little sneared at.
                                                  --Pope.
      [1913 Webster]

   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.
          [1913 Webster]

                The fop, with learning at defiance,
                Scoffs at the pedant and science. --Gay.
          [1913 Webster]

WordNet (r) 3.0 (2006):

sneering
    adj 1: expressive of contempt; "curled his lip in a supercilious
           smile"; "spoke in a sneering jeering manner"; "makes many
           a sharp comparison but never a mean or snide one" [syn:
           supercilious, sneering, snide]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "sneering":
   arrogant, bantering, booing, catcalling, chaffing, clannish,
   cliquish, contemptuous, contumelious, derisive, derisory,
   disdainful, exclusive, fleering, flippant, fooling, grinning,
   haughty, hissing, hooting, jeering, joshing, kidding, leering,
   mocking, panning, quizzical, ragging, railing, rallying, razzing,
   ridiculing, roasting, scoffing, scornful, smart, smart-alecky,
   smart-ass, smirking, snickering, sniffy, sniggering, snobbish,
   snobby, snooty, snorting, snotty, supercilious, taunting, teasing,
   toploftical, toplofty, twitting, withering