This is a fun little French expression that translates directly to being milk soup. Sounds like a compliment? Well, not really. French people use it to describe a person who flies off the handle easily over nothing. A wider meaning would include people who are easily offended or present with sudden mood swings.
The analogy is fairly straightforward: when you heat milk, it boils suddenly and overflowing can only be avoided by moderating the temperature. Moderating their anger is precisely what people with short fuses have a hard time doing. They can't help it: they are soupe au lait. French synonyms include impétueux and sanguin, which also translate to hotheaded.
The emphasis is on how quickly one moves from being calm to blowing a gasket and implies that such a person will revert to being pleasantly normal just as fast. In other words, your chronically cranky neighbor or gout-inflicted, irascible uncle do not quite fall in the milk soup category.
The expression comes from the 19th century expression monter comme une soupe au lait - to go up like milk soup - and finds its origin in good old country-style French cuisine. No surprise here. A soupe au lait is a very old recipe that consists of milk boiled with bread. Something even your cat could have come up with. Alternatively, one could add onions, eggs and butter - ingredients that were more or less always at hand, even in the poorest households, when not much else was left in the pantry. If you were in a really festive mood, you could even add sugar and turn it into a dessert. Looking for a quick and simple recipe for tonight's dinner? This one is hard to beat.
Papa est tellement soupe au lait; il s'énerve toujours pour rien.
Dad has such a short fuse. he always gets angry over nothing.
Lorsqu'une personne est vraiment fatiguée, elle a tendance à être soupe au lait.
When a person is really tired, he/she gets upset easily .