haha

ha-ha An onomatopoeic representation of laughter.
Type of boundary to a garden, pleasure-ground, or park, designed not to interrupt the view and to be invisible until closely approached.
(rfdat), Horace Walpole, 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 Bridgmans) 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.''

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.