The Collaborative International Dictionary of English v.0.48:

Chortle \Chor"tle\, v. t. & i. [imp. & p. p. Chortled; p. pr.
   & vb. n. Chor"tling.]
   A word coined by Lewis Carroll (Charles L. Dodgson), and
   usually explained as a combination of chuckle and snort.
   [Humorous]

         O frabjous day ! Callooh ! Callay !
         He chortled in his joy.                  --Lewis
                                                  Carroll.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

chortle
    n 1: a soft partly suppressed laugh [syn: chortle, chuckle]
    v 1: laugh quietly or with restraint [syn: chuckle, chortle,
         laugh softly]