A "haha" or sunken fence is a ditch with a retaining wall. This setup creates a barrier in the form of an abrupt shift in ground level, effectively separating one area from another. The advantage of using a haha instead of a traditional above-ground fence is that when viewed from the higher ground level, the haha is all but invisible, say editors at Merriam Webster Inc.
A haha was and still is used when a barrier is needed but an uninterrupted view is desired. With a haha, pastures can be separated from formal gardens with no break in visual continuity.The name is more common in British English than in American English. The term has appeared in several forms, including "ha-ha," "ha!ha!," and "hawhaw." The first recorded use in English is from an early 18th century English gardening book, where it appears as "Ah, Ah." According to this source, a person could be having a leisurely walk though a garden and suddenly see the ground about to drop away due4 to a strategically place haha. While teetering precariously, one might gasp "Ah! Ah!" in surprise. It was apparently from this surprised exclamation that the sunken fence, following a change in spelling, came eventually to be called a haha.