Clean as a whistle? Seriously?

An oxymoron, for sure. :)

I just read a couple of paragraphs on search engine and still don't know where it came from or what it means/meant.  I believe it may have originally been used the same as we would say a cut is a clean cut...no threads or leftovers of any kind...not jagged.

That phrase started back in the 18th century ... even before my time!  
It referred to the clean sound a whistle makes.

Clean is a whistle first appeared in the 18th century, meaning"completely, absolutely, leaving no trace. 

The whistle is clean, its the referee holding it that's dirty.