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]

WordNet (r) 3.0 (2006):

ha-ha
    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]