It came as a shock to me that my friend had lost his ___________ in a moment of madness as he lent all of it to his neighbour.

The blank needs an idiom referring to someone's savings or money set aside for the future.

Option 1 -> nest egg -> refers to a sum of money saved for future use, fits perfectly in context.

Option 2 -> nests and eggs -> not an established idiom; literal meaning doesn't fit the context.

Option 3 -> bird's nest -> refers to an actual bird's dwelling, not a financial term.

Option 4 -> nestled eggs -> not a recognized idiom; doesn't relate to money or savings.

A nest egg is money that you've carefully saved up over time, just like a bird carefully tends to its precious eggs in a nest. When someone loses their nest egg, they've lost their financial security blanket - all those hard-earned savings they were counting on for their future.