Something is over and done with; you can’t change the past.
This idiom is akin to the expression: time stands still for no one. It is also akin to and derives from an ancient Greek expression. Heraclitus, a philosopher who lived during the 4th and 5th centuries BCE, coined the phrase, “You cannot step twice in the same river.”
The phrase is used to refer to an event that has already happened and therefore cannot be changed. Consequently, there is no point worrying about it. Time flows in the same way water flows and, once it has passed, there is no reason to dwell on it. What’s done is done.
The earliest citation for ‘water under the bridge’ has a French origin from the 1762 Dictionnaire de l’Académie françoise (Dictionary of French Academy).
There are two citations in the OED:
1913 Wireless World “Much water has flowed under London Bridge since those days.”
1914 Birkenhead re Rudyard Kipling (1978) “…but of course—much water, or shall we say much blood, has flowed under the bridges since they were written.”
And, in 1934, a song entitled Water Under the Bridge, was written by Paul Francis Webster and Lew Pollack and performed by Fred Waring.
Someone who says ‘that’s water under the bridge’ is implying that they’ve moved forward with their life, that the matter has become unimportant and not worthy of further notice. This person has let the past go and, supposing the event was an argument, they are not holding a grudge or harboring bad feelings. The flow of a river is often associated with the flow of life, and the flow of time. The passing of time, water, and life is inevitable.
The expression seems to have become relatively common in theUSA around 1930. However, in America, the proverb is often phrased as, ‘that’s water over the dam.’
Of course, if you fall off the bridge and into the water, and flow along with the current, you are metaphorically living in the past. Which doesn’t seem to me to be a good idea. The present is full of brand new moments. Or brand new stretches of water, which you’ll miss if you’re downstream somewhere in the past. And I think I’d better stop before I confuse myself any further.