As an American with a fascination for all things Regency, I sometimes run across British vernacular that leaves me running for the dictionary. I had just such an experience a short time ago when a discussion among fellow Regency geeks--some of whom are Brits--brought up the word ha-has. Based on the context, it was clear to me they weren't talking about something funny.
In a nutshell, a ha-ha is a short retaining wall used to act as a fence to keep animals out of an area such as a garden or the front lawn of a house.
According to Wikipedia:
A ha-ha (or ha-ha wall) is a recessed landscape design element that creates a vertical barrier while preserving views. The design includes a turfed incline which slopes downward to a sharply vertical face, typically a masonry retaining wall. Ha-ha's are used in landscape design to prevent access to a garden, for example by grazing livestock, without obstructing views. In security design, the element is used to deter vehicular access to a site while minimizing visual obstruction. The name "ha-ha" derives from the unexpected (i.e., amusing) moment of discovery when, on approach, the recessed wall suddenly becomes visible.
I found the name as charming as the concept. You can be sure such a landscape design will find its way into a future Regency romance novel that I write.
Wikipedia article: (http://en.wikipedia.org/wiki/Ha_ha)