the.com/water under the bridge

means Used to say that something unpleasant happened in the past and should now be forgotten or forgiven, because it can no longer be changed.

from A 19th-century image built on simple river logic: water that has flowed under a bridge is already downstream and gone, impossible to call back or undo. The phrase existed as literal description before it hardened into idiom, and it travels alongside the older 'water over the dam,' an American variant born near mills and dams rather than bridges — same current of thought, different scenery.