A branch of obnoxious drivers who pack their cars with ridiculously powerful subs, and blast shitty, overly synthesized, "thumpy" rap music to assert their dominance. In most cases, thumpers can also be referred to as rattlers, due to the intensely low frequency vibrating shaky or loose parts in the car's frame, emitting an even more annoying rattling sound.
Friend 1: "I didn't get any damn sleep. Fucking thumpers kept strolling by my house".
Friend 2: "Dude, I know! Just this morning, I was sitting at a red light, and my rear-viewies were shaking from a thumper two cars behind me!"