What is a fill billy?

There's no commonly known phrase or term called "fill billy." It's possible you've misheard or misspelled a word or phrase.

Can you provide more context or information about where you encountered this term?

For example:

* Was it written or spoken?

* What was the topic of the conversation or text?

* Was it used in a specific region or group?

With more details, I might be able to understand what you're looking for.