nest egg
Definition :an amount of money that you have saved for the time when you stop working, get married etc
Origin :
养鸡户们为了得到更多的鸡蛋，就把真的鸡蛋和假的混杂在一起，放在鸡窝里，目的是使母鸡在同一个地方继续下更多的蛋。所以鸡窝中的蛋就越来越多了，后来西方人就用这个成语来形容储蓄了。
Example:
This investment will be a nice little nest egg for you when you get married.