The Collaborative International Dictionary of English v.0.48:

Guffaw \Guf*faw"\, n.
   A loud burst of laughter; a horse laugh. "A hearty low
   guffaw." --Carlyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guffaw
    n 1: a burst of deep loud hearty laughter [syn: guffaw, belly
         laugh]
    v 1: laugh boisterously [syn: guffaw, laugh loudly]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "guffaw":
   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, gales of laughter, giggle, go into convulsions,
   ha-ha, hearty laugh, hee-haw, hee-hee, hilarity, ho-ho, horselaugh,
   laugh, laugh it up, laugh outright, laughing, laughter,
   nearly die laughing, outburst of laughter, peal of laughter,
   risibility, roar, roar of laughter, roar with laughter,
   shake like jelly, shake with laughter, shout, shout of laughter,
   shriek, snicker, snigger, sniggle, snort, split,
   split with laughter, tee-hee, titter, yuck, yuk-yuk