n. An adult son or daughter, particularly one aged 30 or more, who still lives with his or her parents.
Other Forms
Have you got a kipper in your nest? The first withering acronym of the year has been coined by the British building society Prudential to describe adult children reluctant the fly the nest. "Kippers" — Kids In Parents' Pockets Eroding Retirement Savings — are a million strong in Britain. In Japan, where they are described as "Parasite Singles", adults living in the family home are considered a serious drain on the ageing population.
—Louise Holden, “Have you got a kipper in your nest?,” The Irish Times, January 13, 2004
Of those adult children who live at home, 30% don't work at all and 49% are in full-time employment. Of those in work, 58% live at home rent-free and only one-third are charged rent, averaging at (pounds) 95 per month. While this may be a testimony to good parenting skills, now that raising our children is taking on epic proportions it is important that the communal living space should work smartly around the generations. These three properties all come with strong kipper potential which demands generously proportioned rooms, a hub-of-the-house kitchen and ample parking space.
—Heather Macleod, “3 of a kind,” The Herald, December 03, 2003
2003 (earliest)
The combination of high property prices and laziness has left many parents with the surprise — one that is not always welcome — of finding their thirtysomething children are not in a rush to leave.

And who are they? They are "kippers", an acronym for "kids in parents' pockets eroding retirement savings", according to the survey from the Prudential, the financial services group.
—Becky Barrow, “Parents pay up as 'kippers' refuse to sever home ties,” The Daily Telegraph, November 17, 2003
This word is usually seen in plural form since the phrase on which it's based — kids in parents' pockets eroding retirement savings — works best with "kids" rather than "kid." In any case, it all feels a bit forced, but the media have lapped it up, so the word may be here to stay (at least in countries that are familiar with the traditional meaning of kipper: a smoked herring, often eaten at breakfast).

Silly as this acronym may be, the image it conjures up of children taking their parents' retirement savings illustrates that at least some parents resent their children hanging around the nest. This is also evidenced by some of the synonyms for such children, including parasite singles and slops: singles living off parents.