It’s a word, believe it or not, and it comes from the French exclamation of surprise : Ha ha! (according to all the dictionaries I consulted). It designates a ditch that serves as a boundary without obstructing the view. Presumably, it is so unobstrusive that you only see it when you’re up close, hence the surprise. Here is an example: the ha-ha in Greenwich.
What I’d like to know is why it was named after a French exclamation? Is it just that the French, being more expansive, expressed their surprise when encountering this odd construction while cool British people kept their reserve and didn’t react? The other point I’d like to make is that not many French people I know would say Ha ha! when surprised… Ah! Maybe, or Oh!, but not Ha ha! Very odd indeed.
I’m off interpreting this morning for the last meeting of the two-year project I’ve been involved with. I’ll ask the French project partners what they think about this particular pressing issue over dinner.