It cannot be changed. The expression comes from the idea that water that has fallen over a dam cannot be brought back again.

VOA: special.2009.05.10

People who hold constrained vision of human nature, believe that human nature cannot be changed; it's immutable-- we have certain instincts, we have certain inclinations.

认为人性受约束的观点的人，相信人性无法改变;,它是永恒的，我们有一些本能，有一些欲望。

哈佛公开课 - 幸福课课程节选