nest egg

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. An egg (natural or artificial) placed or left in a nest to prevent a laying hen from forsaking the nest.
2. n. Something laid up as the beginning or nucleus of a continued growth or accumulation.

Wiktionary

1. n. A natural or artificial egg placed in a bird's nest, to encourage the bird to lay its own eggs there.
2. n. idiomatic A savings; a reserve of money.

GNU Webster's 1913

1. n. An egg left in the nest to prevent the hen from forsaking it, and to induce her to lay more in the same place.
2. n. Money set aside in a special fund to be used for special occasions, or more commonly, for retirement.

WordNet 3.0

1. n. a fund of money put by as a reserve
2. n. device consisting of an artificial egg left in a nest to induce hens to lay their eggs in it