Source : Webster's Revised Unabridged Dictionary (1913)

Haw-haw \Haw-haw"\, n. [Duplication of haw a hedge.]
   See {Ha-ha}.

Source : WordNet®

haw-haw
     n 1: a loud laugh that sounds like a horse neighing [syn: {hee-haw},
           {horselaugh}, {ha-ha}]
     2: a ditch with one side being a retaining wall; used to divide
        lands without defacing the landscape [syn: {sunk fence}, {ha-ha}]