nest egg

noun
  1. 1

    A natural or artificial egg placed in a bird's nest, to encourage the bird to lay its own eggs there.

  2. 2

    A saving; a reserve of money.

    โ€œOver time, they accumulated a tidy nest egg and retired comfortably.โ€

  3. 3

    An egg squash.

Source: https://en.wiktionary.org/wiki/nest%20egg