[1] : The Collaborative International Dictionary of English v.0.48

ha-ha \ha-ha"\ haumhaum", 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
see also:
haw-haw haw-haw 

[2] : WordNet (r) 2.0

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

Haha >> H Words
Haha, definition of term: Haha
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