Alternative forms 
Etymology 1 
Etymology 2 
French haha. French term attested 1686 in toponyms in New France (present Quebec); compare modern Saint-Louis-du-Ha! Ha!. Usual etymology is that an expression of surprise – “ha ha” or “ah! ah!” is exclaimed on encountering such a boundary. In France this is traditionally attributed to the reaction of Louis, Grand Dauphin to encountering such a feature in the gardens of the Château de Meudon. English term attested 1712, in translation by John James of French La theorie et la pratique du jardinage (1709) by Dezallier d'Argenville:
Grills of iron are very necessary ornaments in the lines of walks, to extend the view, and to show the country to advantage. At present we frequently make thoroughviews, called Ah, Ah, which are openings in the walls, without grills, to the very level of the walks, with a large and deep ditch at the foot of them, lined on both sides to sustain the earth, and prevent the getting over; which surprises the eye upon coming near it, and makes one laugh, Ha! Ha! from where it takes its name. This sort of opening is haha, on some occasions, to be preferred, for that it does not at all interrupt the prospect, as the bars of a grill do.
- Type of boundary to a garden, pleasure-ground, or park, designed not to interrupt the view and to be invisible until closely approached.
- Horace Walpole's essay On Modern Gardening: The Ha Ha But the capital stroke, the leading step to all that, has followed, was (I believe the first thought was Bridgman's) the destruction of walls for boundaries, and the invention of fosses - an attempt then deemed so astonishing, that the common people called them Ha! Ha's! to express their surprise at finding a sudden and unperceived check to their walk. One of the first gardens planted in this simple though still formal style was my father's at Houghton. It was laid out by Mr. Eyre, an imitator of Bridgman. It contains three-and-twenty acres, then reckoned a considerable portion.
- Richard Bradley, New improvements of planting and gardening, both philosophical and practical, London, 1731, p. 164: Haha! or Fossee, are Terms of the same Signification, tho' the First is a new coin'd Word, they mean a Ditch, or Moat to Enclose a Garden, whether the Ditch has Water in it, or not, but the Haha, by the Custom of five or six Years, intimates a dry Ditch, so regulated by Slopes, and so Deep that it is unpassable. It makes a fine open Fence to a Ground.
- ha-ha (ditch acting as a sunken fence)
- See はは
Rapa Nui