What does the cowboy slang a mean?

"A" is not a specific slang term in cowboy culture. It's likely you're thinking of a different term, or it's a very localized expression. Could you provide more context or a specific example of how the term "a" was used?

For example, you might be thinking of:

* "A-okay": meaning "all right" or "very good".

* "A-horseback": meaning "on horseback".

* "A-runnin'": meaning "running" or "moving quickly".

If you can give me more information, I might be able to help you understand the specific meaning of the term you are looking for!