Skunk biscuit
Similar to a shart, this happens when you think you need to fart, but a small biscuit of poo tags along AND leaves a dark streak on your underwear. Named for its smell, color pattern, and general consistency. These are common among older people who have lost control of their sphincter.
(Clinching butt cheeks together while walking stiff legged) Oh, shoot! I'm gonna need a bathroom right quick, I've got a skunk biscuit here. These Wednesday undies are definitely going in the trash.