What does on a mean in slang?

"On a" is not a common slang phrase in English. It's possible you've heard it used in a specific context or dialect that you're unfamiliar with.

To understand what it might mean, please provide more information:

* What was the context? Was it a conversation, a song lyric, a written text, etc.?

* Who said it? Was it a friend, a family member, someone from a particular region, etc.?

* Can you give an example sentence? This will help me understand the intended meaning.

With more details, I can hopefully figure out what "on a" means in your particular situation.