Literally - "race bread roll"
Figuratively - The German language equivalent of a rice burner or "tricked out whip". Usually used to denote a hatchback or small automobile with tasteless street racing modifications. Popularized by upper-crust BMW drivers exercising their Autobahn elitism.
Check out that VW Polo with the huge ass wing and exhaust What a fugly rennsemmel, I bet it only makes 50 whp...