It comes from the biblical verse in Ecclesiastes 11:1: "Cast your bread upon the waters, for you will find it after many days." This verse suggests that while you might not see the immediate results of your generosity, it will ultimately be rewarded in some way.
Here are some other ways to interpret the idiom:
* Giving without expecting a return on investment.
* Acting selflessly.
* Having faith that good deeds will be rewarded, even if not immediately.
* Not expecting gratitude or recognition for your kindness.
The idiom emphasizes the importance of being generous and doing good for its own sake.