Definition: Money or investments set aside for future use.
Example: Ned had been saving up a nest egg for decades, for an early retirement. Or a trip to Vegas.
Farmers have been putting eggs in chickens’ nests for centuries. Now, that sounds wrong. Surely Iddy means that farmers have been taking eggs from chickens’ nests for centuries. Yes, they have been doing that as well. Putting an egg, whether real or ceramic, in a chicken’s nest encourages it to lay more. Strange but true.
So, our idiom ‘nest egg’ refers to that practise, putting something aside in order for it to grow for future collection. It first appeared in print with that meaning in 1686.