Ha Ha(1712): this 18th-century garden feature consisted of a trench, the inner side perpendicular and faced with stone, the outer sloping and turfed, that was intended to allow the landowner an uninterrupted view of the countryside. It came from the French word haha �an obstacle interrupting one's way sharply and disagreeably, a ditch behind an opening in a wall at the bottom of an alley or walk�. According to French etymologists the ha! is an exclamation of surprise � or could it be fear?
Well, as we now know its a sunken (hidden) boundry, which was designed to make ones estate look far bigger than it actualy is. when guests would look out from the mansion, it would appear that the grounds extended way out into the surrounding farmland or countryside. it would only become apparent when a guest explored the grounds further and seeing it at close quarters would exclaim ha ha, so thats the boundry down there !