"{one's} luck [is] in" 的定义和意义

What is the origin of the idiom "one's luck is in" and when to use it?

The phrase "one's luck is in" is an informal idiomatic expression that signifies a period of good fortune or a favorable streak. The origin of this phrase is rooted in the notion that someone's luck has improved or is currently on the upswing. It is often used to describe a situation in which things are going well for an individual, typically in a casual or conversational context.