Absolutely exhausted after, say, a very rough day of work? “I’m worn slap out,” is what you would say to show just how beat you are.
Unfortunately, not much on the origin of the phrase is known. It may be just one of those magical pieces of slang that gets passed down from generation to generation, a given phrase that becomes entrenched in a region.
That’s what makes many Southern phrases so beautiful. They’re weird, but they’re unique. Some don’t make sense, no matter how long you study them… but then once you become familiar with them, they’re a part of you. So the weird Southern expressions, actually, are examples of just how beautiful language is and can be.