“A ha-ha is a recessed landscape design element that creates a vertical barrier while preserving an uninterrupted view of the landscape beyond… The name “ha-ha” may derive from the unexpected (i.e., amusing) moment of discovery when, on approach, the vertical drop suddenly becomes visible. “
Kinsmen and Kinswomen 
“A ha-ha is a recessed landscape design element that creates a vertical barrier while preserving an uninterrupted view of the landscape beyond… The name “ha-ha” may derive from the unexpected (i.e., amusing) moment of discovery when, on approach, the vertical drop suddenly becomes visible. “