Also "haw haw
A landscaping term for a fence set in a ditch around a garden, so as to provide a barrier, but not obscure the view. Typically used only by United Kingdomites.
from The Oxford English Dictionary.
In Edward Gorey's The Awdrey-Gore Legacy a ha ha is listed first amoungst the places in the English Garden in which one might find the victim of a murder mystery.
* reprinted in Amphigorey Also.