The effect produced when a car bumps its sound system loud enough to shake the whole car, and rattle the trunk. Most common in shitty Hondas and Toyotas, driven by douchebags that try to be gangsta or cool.
1. Another way to say "idiotic". (If stupid peoples' skulls were coffee cans, you would be able to hear new thoughts rattling around in the empty space where their brains should be.)