A greasy stain that usually appears on one of your shirts after being washed without pre-treating it. Source of the stain is unknown, but the culprit is most likely from the greasy chicken you had last night or from the Vaseline or lotion you used to, well, you know, and you didnt wash your hands well enough. Of course, you're gonna blame the chicken. In either case, if the spot doesn't come out, you throw the shirt away.
Mom: "Son, what's that stain on your shirt?"
Me: *blush* "Oh! I was wearing this the other night at supper when we had fried chicken and I guess I wiped my hand on my shirt. It's a mystery stain, mom."