A beautiful term for an extremely Foul smelling fart. The term is made funnier if the fart sounds like quacking duck when expelled from the rectum. Though an SBD can still qualify as a Butt Duck if it is especially foul (emphasis on the word foul/fowl for comic effect). See frequently activated rectal tremble and shit flavored air.
"Fuck Mary, you shit a butt duck! I think you have cancer, something has crawled up your arse and died!"
by Keifermail August 08, 2009