Hi Frag, it is a proverb. The full saying would be "嫁出去的女儿， 泼出去的水", which means "married daughter is like splashed water (that will never come back)". In old days, once a girl is married, everything of her is considered belongs to her husband and the new family; she no more belongs to her parents' family. That's how this metaphor came from. Though this thought has changed much nowadays, the metaphor is still in use.

We usually use the metaphor "泼出去的水" to describe something that can not be retrieved.