plural of 'hah,' an exclamation or laugh; also, a dialectal or historical term for trenches or sunken fences used in landscape design.
The exclamation 'hah' is imitative (onomatopoeia), while the landscape meaning comes from French 'ha-ha,' describing a ditch or fence sunk below ground level so as not to obstruct views.
The 'ha-ha' fence was a brilliant 18th-century invention—you'd walk to the edge of a garden and suddenly laugh ('ha-ha!') when you discovered an invisible boundary, creating the optical illusion of endless landscape.
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