The English phrase, water under the bridge, is a metaphor for “a conflict in the past, which should no longer be a concern.”
Imagine standing on a bridge over a river, looking at the water pass under you. Once that water has passed, it cannot come back to you; it is gone forever. There is nothing you can do, so it is unwise to worry about it.
Additionally, bridges are meeting places that join two sides together, and so bridges symbolize the resolution of interpersonal conflicts.
Although the idiom, water under the bridge, could stand for any kind of problem, it usually refers to bad feelings between people. Note that water is often a symbol for emotions. When someone uses this phrase, they are saying, “let’s forget about our conflict and bad feelings; they are like water that has already passed under the bridge.”
Nobody is certain of the phrase’s origins, but the oldest example in English may be the song Water Under the Bridge, which was released and became popular in America and Britain in 1934. Ironically, however, in that song, water under the bridge symbolized love flowing through the singer’s heart. Which suggests that English speakers at that time were not familiar with its modern meaning. But the phrase then appeared again in a 1935 newspaper article, with the meaning we know today.