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1.
A vehicle containing a belligerent stereo system with subwoofers that render the trunk useless and broadcast low frequencies for hundreds of yards. The "music" itself is designed to maximize bass, not musicality.

These vehicles tend to be associated with criminal activity and go well beyond a simple nuisance. The drivers are generally mouth-breathing, shaved-headed idiots who'll pick a fight with anyone who wants to relax in their own home.
Another fire truck hit a boom car at an intersection because the driver couldn't hear the siren. Unfortunately, the driver lived.
by Noise-Free March 18, 2010
 
2.
A car driven by a Baby Boomer that is (or will soon become) a hazard on the road.
So named, because the person is too old to drive and will result in a crash "Boom"!
While driving on the Freeway, we saw a Boom Car being driven erratically and it wound up in a crash.
by NoahCG February 03, 2009