The Collaborative International Dictionary of English v.0.48:

Ha-ha \Ha-ha"\ (h[aum]*h[aum]"), n. [See Haw-haw.]
   A sunk fence; a fence, wall, or ditch, not visible till one
   is close upon it. [Written also haw-haw.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Haw-haw \Haw-haw"\ (h[add]*h[add]), n. [Duplication of haw a
   hedge.]
   1. See Ha-ha.
      [1913 Webster]

   2. a loud laugh that sounds like a horse neighing. [WordNet
      sense 1]

   Syn: hee-haw, horselaugh, ha-ha.
        [WordNet 1.5]

   3. a sunken fence (so as not to interfere with the view).
      [WordNet sense 3]

   Syn: haha.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Hawhaw \Haw*haw"\, v. i. [Of imitative origin.]
   To laugh boisterously. [Colloq. U. S.]
   [1913 Webster]

         We haw-haw'd, I tell you, for more than half an hour.
                                                  --Major Jack
                                                  Downing.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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