Someone who drives a compact car (most likely a Honda Civic or Acura Integra) that sports a variety of tacky looking aftermarket parts which are meant to add visual flare and improve the car's performance. The most notable feature is the car's huge muffler that looks like a coffee can, and makes it sound like a weedwacker. Other common bolt-ons include huge spoilers, body kits, hood scoops, gauges, window stickers and other decals, tinted windows, lowered suspension, bright colored paint job, etc. More often than not, these cars aren't even that fast and are usually driven by high-school kids.
The sound coming from that muffler monkey's car is really irritating.