word of the week: ha-ha

Ha-ha (n.) A sunk fence; a fence, wall, or ditch, not visible till one is close upon it.

You have to wonder a bit about the etymology. Is it 'ha-ha' as in "Yes, very clever, now help me up because my ankle's twisted and there's no way I can make it back to the manor on my own," or as in "Oh look, another trespassing idiot didn't look where he was going"?