Literally translated as, putting water in one’s wine, the expression means toning it down, deciding to adopt a more moderate stand on an issue or in an argument, being more tolerant, making an effort to reach a compromise. It is mostly used in a positive sense.
The idea is that pouring a little water into a glass of wine makes it milder in both flavor and alcohol content, and therefore easier to drink. Likewise, in the figurative sense, someone who cuts his wine with water makes his views easier to accept, or agree to.
Note that the expression could also mean LITTERALLY pouring water in one's glass of wine. in most wine connoisseur's circles, however, the act would be frowned upon and seen as an offense to good taste these days.
The expression dates back to 1576, although it had a different meaning originally. At the time, adding water to one's glass of wine was seen as an attempt to moderate one's anger. The focus was therefore more on calming oneself down rather than trying to reach a compromise. The tradition it refers to is even older. In Ancient Greece and Rome, adding water to wine was all but required. Not doing so was considered low class. The purpose was to avoid getting drunk - a sure sign of low class behavior. Nowadays the notion of calming one's temper has been completely replaced with that of lessening one’s demands or ambitions in order to reach a compromise.
Ils ont été fâchés pendant des années, mais il a mis de l’eau dans son vin et leurs relations sont plus sereines maintenant.
They had been mad at each other for years, but he met her halfway and their relationship is calmer now.
Au début, elle ne voulait pas que son fils joue aux jeux vidéo, mais elle a mis de l'eau dans son vin et maintenant il a le droit de jouer le weekend.
Initially, she didn’t want her son to play video games, but then she backed down and now he’s allowed to play on weekends.