Definition of 'Boomerang'
An American slang term that refers to an adult who has moved back in with his or her parents (who are part of the baby boomer generation) instead of living independently. The phrase, when applied to an individual, makes reference to the fact that the person lived independently for a period, but subsequently returned home due to the financial costs associated with maintaining a separate household.
Investopedia explains 'Boomerang'
While boomer parents may be pleased emotionally to have their boomerangs back in the household, boomerangs can often pose a significant financial burden on their parents. This can result in a reduction in retirement savings for the boomerang's parents, leaving them with the decision to either postpone their own retirement or have their children help out with the household expenses.
Other countries have adopted similar slang to represent this domestic phenomenon. In Italy, the term "mammon", or "mama's boys" is used, while the Japanese refer to them as "parasaito shinguru", or "parasite singles". In the U.K., children boomeranging back home has given rise to the acronym KIPPERS (or kids in parents' pockets eroding retirement savings).