An idiom is a figure of speech that is commonly used and is well known enough that most people will understand its meaning. It is usually a simile or a metaphor.
One such idiom is “a slip of fate.” Another variation of this is “a twist of fate.”
The girl was one of those pretty and charming young creatures who sometimes are born, as if by a slip of fate, into a family of clerks.
This idiom means that we are headed in one direction, and then fate sends us in another direction. “Fate,” of course” being a concept meaning that something was meant to happen. In this case, Mathilde was meant to be wealthy, but due to fate, she ended up born to a clerk. This left her pining for the life she felt she should have had.
Another idiom is to say that she was “made for” a better life. To say that a person is made for something is to imply that a person is well suited to something. This goes along with the feeling that Mathilde should have been born into a rich household, if not for the slip of fate. She was “made for” better things.
She had no gowns, no jewels, nothing. And she loved nothing but that. She felt made for that. She would have liked so much to please, to be envied, to be charming, to be sought after.
Mathilde is unhappy with her perceived poverty, even though she is closer to middle class, and feels as if she is “made for” this lifestyle of gowns and jewels. The idiom works because it implies that she feels as if she was born for a better life, and deserves it.
When Mathilde asks her husband for money, we see two more idioms: “laying aside” and “treat himself.”
He grew a little pale, because he was laying aside just that amount to buy a gun and treat himself to a little shooting next summer on the plain of Nanterre, with several friends who went to shoot larks there of a Sunday.
These are common idioms still used today. To lay aside money means to save it, usually to save up for something. The idiom “treat himself” refers to the fact that he was planning to buy himself something that cost a lot of money that he needed to save up for. You treat yourself by sacrificing by saving up money. They do not have much money, so he needed to save over time in order to buy the gun. He sacrifices even more when he decides instead to give the money to his wife to buy a dress. In some ways this is less practical, because you can use a gun for years, but a dress can’t be worn many places or as often.
These are the best idioms from the story. You will also find “catch cold,” but I do not need to really explain that one.
The use of idioms is a good way for the author to convey interesting thoughts and make a point in a way that will be meaningful to the reader, and still tell the story. Most of these idioms show how unhappy Mathilde is with her fate. She wants a different life than the one she was born into.