1. When you are stuck in traffic and you take a shit in a cup and stash it under your seat.
2. A ho you bring on a long road trip.
"Damn i left a road muffin in my car and now it smells like shit!"
"Jenny's my road muffin, she gave me head all the way to Denver."
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The female equivalent of road head.
Man, I'm glad she wore a skirt so I could give her a little road muffin on that long drive.