Definition & Meaning of "upper crust"in English

What is the origin of the idiom "upper crust" and when to use it?

The idiom "upper crust" originated in the late 19th century and early 20th century, primarily in the United States and Britain. It emerged as a metaphorical expression to describe the highest social class or the elite members of society. It is often employed to discuss or critique issues related to social class, inequality, and elitism.