Definition & Meaning of "[know] what [is] what"

What is the origin of the idiom "know what is what" and when to use it?

The origin of the idiom "know what is what" is not well-documented, but it is part of a broader group of idiomatic expressions related to knowledge and understanding. The use of "what is what" in this context suggests a clear understanding of the true nature or essence of something. This phrase is used to emphasize that someone possesses a deep understanding of a particular subject, situation, or set of circumstances. It implies that the individual can distinguish between what is true or important and what is not, making informed decisions or judgments.