- a state of being
Being familiar with a topic or a broad range of topics. Either having extensive knowledge in a subject that your peers would consider you an expert, or having a firm grasp of a variety of topics that can summarize a wealth of practical knowledge.

For many, especially in the Southern US, it means the person referenced is an ignoramus . That is, if the person had their shoes shined. they could easily be fooled by the shine man who could just as well put shit on their shoes as shoe polish

An extreme way to invalidate a person's opinion. It's a hardcore dis meant to bring the person it's directed at to complete silence.

To know your shit means you're really knowledgeable about a particular topic or field

Do they know their shit means have they mastered something, or do they know they're shit meaning their ignorance makes them worthless as an expert.

To be oblivious or uninformed on a subject you are trying to create a dialogue about.

Meaning that someone is very good at what he does, and knows a lot about it (basically the exact thing as "Knows his stuffs)