Used as an insult and to imply anything really nasty or extremely gross. Can also be used in place of the word shit and it's various forms, and is limited only to your imagination as to how it is used.
She wears so much makeup her face looks like butt drool.
His breath smells like butt drool.
You're butt drool.
Quit your butt droolin' and get me a beer.
Don't gimme any more of your butt drool.
Man, I feel like butt drool.