h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This place-name was mentioned recently in (I believe) National Geographic. There a more plausible (to me) derivation was put forward involving a haha. A haha is a sudden drop or ditch designed to create a barrier to wildlife, but without having an ugly wall blocking the view from your stately home. Nat G. states the word as obsolete French, which it may well be, but it also current (if specialised) English.

Byron

The term haha is well recognised in the North of England as a sort of retaining wall, level with the ground on the upper side but the ground level much lower on the other side. You have to look where you are going so as not to fall off.
There are many examples in National Trust properties - Lumley castle in County Durham springs easiest to my mind!

I am reminded of the Terry Pratchett character 'Useless' Jones (a parody on 'Capability' Brown) who built 'Ho-Ho' - defined as 'just like a haha - only deeper...'

There's a thing about Ha Has in the current issue of the National Trust newsletter.
They say it's a ditch - but the object was to keep cattle off the front lawn of your stately home!