Water under the bridge

    过去的事了

    A: Mary used to have a crush on the boy next door.

    B: She actually went out with him for some time but that's all water under the bridge. She married someone else long ago.

    Note: This is an idiom cliche used to refer to something that is over and gone and therefore not worth thinking any more about. It dated from the 20th century and is still widely used. The proverb comes from the saying "A lot of water has flowed over the dam (or "under the bridge"). "Over the dam" is its American variant.