The song seems to be all about having EXTREMELY dirty anal sex and being high on codine ... yeah, not my thing
Here's the lyrical meaning:
"Old town road" - A VERY VERY old term for a VERY disgustingly dirty hole of an ass being discovered during anal sex. It's not in the Urban Dictionary.
"Horses in the back" - https://www.urbandictionary.com/define.php?term=horse+back+sex
"Lean all in my bladder" aka codine - https://www.urbandictionary.com/define.php?term=leanin
"Ridin' on a tractor" - https://www.urbandictionary.com/define.php?term=riding%20the%20tractor
"Bull ridin' and boobies" - https://www.urbandictionary.com/define.php?term=bull+riding