A reader writes:
Obviously, you have never kept chickens! A “nest egg” is an egg that is not collected, but instead, left where the human poultry-keepers want the hen to lay more eggs. The nest eggs keep the hens returning to the same spot to add one more egg. Nest eggs nowadays are usually fake (Golf balls are often used.) but traditionally they were just eggs that one delayed collecting for a few days. So, a “nest egg” means a consumable good that one defers consuming (and, like interest-earning money in the bank, can help one accumulate more).
BTW, a number of everyday terms come from poultry-keeping. A low-quality item (like stale bread given to chickens) is “for the birds”. A “brooding” person is inactive and withdrawn from regular activities, like a hen sitting on eggs to hatch. “Hackles” are a chicken’s neck feathers, and a chicken with its "hackles up” is an angry one (as is a human).