nest egg

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fund of money put by as a reserve
• noun device consisting of an artificial egg left in a nest to induce hens to lay their eggs in it