nest egg

1. An artificial or natural egg placed in a nest to induce a bird to continue to lay eggs in that place.
2. A sum of money put by as a reserve.

A special sum of money saved or invested for one specific future purpose.

Investopedia Says:
Examples of the purposes for which nest eggs are usually intended include retirement, education, and even entertainment (vacations and cruises). The main idea is that the money in the nest egg shouldn't be touched except for the purpose for which you saved it.

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Assets put aside for a person's retirement. Such assets are usually invested conservatively to provide the retiree with a secure standard of living for the rest of his or her life. Investment in an Individual Retirement Account would be considered part of a nest egg.

noun

A supply stored or hidden for future use: backlog, cache, hoard, inventory, reserve, reservoir, stock, stockpile, store, treasure. Slang stash. See collect/distribute.