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]