Betcha didn’t know that "ha-ha" is an actual word meaning a trench, basically a sunken fence to keep livestocks from coming into your backyard while preserving the uncluttered look of the landscape:

… the name is derived from the response of ordinary folk on encountering them and that they were, "…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."

More on Ha-ha at Wikipedia – via Fancy Notion’s Word of the Day