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.
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.
... funny little drawling speech about the needs of her pet department. Two or three of Miss Ferris's admirers declared that zooelogy was the most important subject in the college curriculum, and urged that the money should be used as a nest egg for endowing the chair occupied by that popular lady. The Spanish and Italian departments, being newly established, were suggested as particularly suitable objects for benevolence. Dr. Hinsdale's department, the history and the Greek departments were exploited. 19— was a versatile class; there ... — Betty Wales Senior • Margaret Warde