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]