A Muffler with an excessively and impractically large diameter output pipe (or an excessively large tailpipe on cars with mufflers located further from the rear of the car) - especially one that is mated to a much smaller upstream exaust system. Typically used to make a car "look fast" as opposed to modifications that actually make a car faster.
This type of exaust is called a "mellon shooter" because it is large enough to launch a melon that would be inserted into it - if the engine had enough power.
Look at the melon shooter on that Civic - it's as big as a sewer pipe!
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