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.