What does the cowboy slang a mean?

"A" is not a common cowboy slang term. It is possible that you are thinking of a different word or that the term is very specific to a certain region or group of cowboys.

To help me understand what you are looking for, could you provide more context? For example:

* Where did you hear this term? Was it in a book, movie, or conversation?

* What was the context of the term? What were the people talking about?

* Can you give an example sentence of how the term was used?

With more information, I can help you figure out what "a" means in cowboy slang.