origin: said to be French (18th century), from the exclamation made when one accidently discovers one!
A sunken fence or boundary marker, particularly used within British estates & murder mysteries.
Livestock, usually sheep, were utilized for grazing before the common use of electric lawnmowers; a "ha-ha" allowed them to feed on large estates, while keeping them off the lawn and gardens adjoining the house without a visible barrier -- thus allowing an unobstructed view as well!
* In the Terry Pratchett Discworld novel Men at Arms, a similar landscape boundary is used for a comedic twist: designed by ill-famed engineer Bergholt Stuttley Johnson, the ha-ha is accidentally specified to be 50 feet deep, is called a "ho-ho", and is reported to have claimed the lives of three gardeners.
* In Pratchett's book with Neil Gaiman, Good Omens a character in the book is said to be lying face down in the ha-ha, but not to be very amused by it.
* The Island of Doctor Moreau by H. G. Wells, a "steep-walled gap" is compared to "the ha-ha of an English park."
* Edward Gorey's The Awdrey-Gore Legacy, a satire on overcomplicated murder mysteries, ha-ha is one of the typical places where the body of a murder victim might be found
* In Tom Stoppard's Arcadia, the ha-ha is discussed in relation to the Capability Brown garden and is used as one of the links between the nineteenth and twentieth century characters
* Jane Austen's Mansfield Park a ha-ha prevents more sensible characters from getting around a locked gate and into woodland.