It used to be slang for cocaine when I was in high school but slang changes over time and with location, so I don’t know if that’s still the case for this particular instance.
It’s definitely an Appalachian thing, even earlier than Turtle Man you could hear people hauling ass in their dilapidated overgrown F350s and shouting “yeeyee” from the driver’s seat.
As for a definition, it’s like the mountain version of “yeehaw” or “let’s get this bread”
I have no idea and I’ve never heard of such a thing, but this is from urban dictionary:
An exclamation used by hunters, fishers and other country folk when just about anything happens. It’s used under any circumstances usually with the phrase “live action” added in before. Hunters will occasionally raise their guns in the air to exclaim a yee yee. (Yee may be repeated more than twice if appropriate.)