The Collaborative International Dictionary of English v.0.48:

nest egg \nest egg\, n.
   1. An egg left in the nest to prevent the hen from forsaking
      it, and to induce her to lay more in the same place.
      [1913 Webster]

   2. Hence, (figuratively), Money set aside in a special fund
      to be used for special occasions, or more commonly, for
      retirement; as, she set aside a large hunk of her salary
      every month and had a considerable nest egg for her
      retirement.
      [PJC]

WordNet (r) 3.0 (2006):

nest egg
    n 1: a fund of money put by as a reserve [syn: savings, nest
         egg]
    2: device consisting of an artificial egg left in a nest to
       induce hens to lay their eggs in it

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "nest egg":
   Swiss bank account, assets, backlog, balance, bank account,
   bottom dollar, budget, cache, cash reserves, checking account,
   command of money, exchequer, finances, fund, funds, hoard,
   inventory, kitty, life savings, means, moneys, pecuniary resources,
   pocket, pool, purse, reserve, reserve fund, reserve supply,
   reserves, reservoir, resource, resources, savings, savings account,
   sinking fund, something in reserve, stock, stockpile, store,
   substance, treasure, unexpended balance, unregistered bank account,
   wherewithal