The Collaborative International Dictionary of English v.0.48:

Snicker \Snick"er\, v. i. [imp. & p. p. Snickered; p. pr. &
   vb. n. Snickering.] [Cf. D. snikken to sob, to sigh.]
   [Written also snigger.]
   1. To laugh slyly; to laugh in one's sleeve.
      [1913 Webster]

   2. To laugh with audible catches of voice, as when persons
      attempt to suppress loud laughter.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Snicker \Snick"er\, n.
   A half suppressed, broken laugh. [Written also snigger.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

snicker
    n 1: a disrespectful laugh [syn: snicker, snort, snigger]
    v 1: laugh quietly [syn: snicker, snigger]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "snicker":
   Homeric laughter, be in stitches, belly laugh, boff, boffola,
   break up, burst into laughter, burst of laughter, burst out,
   burst out laughing, burst with laughter, bust a gut, cachinnate,
   cachinnation, cackle, chortle, chuckle, convulsion, crow,
   fit of laughter, fleer, gales of laughter, giggle,
   go into convulsions, guffaw, ha-ha, hearty laugh, hee-haw, hee-hee,
   hilarity, ho-ho, horselaugh, jeer at, laugh, laugh at, laugh it up,
   laugh outright, laughing, laughter, leer, mock,
   nearly die laughing, outburst of laughter, peal of laughter,
   risibility, roar, roar of laughter, roar with laughter,
   sardonic grin, scorn, scornful laugh, shake like jelly,
   shake with laughter, shout, shout of laughter, shriek, smirk,
   sneer, snigger, sniggle, snort, split, split with laughter,
   tee-hee, titter, yuck, yuk-yuk