The liquid that is held between the cheeks of your ass that smells like a mixture of sweat and shit.
Person 1: Dude, I was sitting down all day, and I've accumulated some Bum Sauce
Person 2: EEEWW!! THAT'S GROSS! AIN'T NOBODY WANNA HEAR ABOUT YOUR BUM SAUCE!!
A term for when a person does something so incredibly dumb in the attempt to be funny or do something stupid for the hell of it, actually succeeds in the task even if it wasn't possible.
Friend #1: *flies off a cliff in a video game and doesn't take fall damage*
Friend#2: What?! how did you not die?
Friend#1: gotta be Bum's Luck haha