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

The origin of the idiom "know one's stuff" is derived from the word "stuff" in the sense of knowledge, expertise, or skills. It is a colloquial expression that has been used to describe someone's proficiency or competence in a particular field or subject matter. This expression is used to convey that someone possesses a deep understanding and expertise in a specific area or topic. It is often employed to commend or acknowledge a person's knowledge and competence, especially in professional or academic contexts.