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 ]