What is the origin of idiom split whistle?

The idiom "split whistle" is not a commonly used idiom. It's possible that you've misheard or misinterpreted something.

Here are some possible scenarios:

* A misheard phrase: You might have heard a similar phrase like "split second" or "split hairs."

* Regional slang: The phrase might be specific to a particular region or social group.

* A made-up phrase: It's possible the phrase is a humorous invention or a made-up phrase.

If you can provide more context about where you heard this phrase, I might be able to help you understand its origin.