The act of inserting mustard into a vagina or anus, then inserting a penis or large piece of meat into the vagina. The piece of meat could be a bratwurst, hot dog, or even a steak if you're feeling extra meaty.
It gets it's name from the creator, Michelle. I'm sure it would work well with anyone, although I hear it's better with a Michelle.
Male A: I'm in the mood for a hot dog. That sounds delicious.
Male B: I agree, although Michelle's Mustard Dog sounds even better. What is her phone number?
Male A: I have no idea. Besides, I'm more of a ketchup guy myself.