i've seen that one too; it's also meant to convey, imho, i'm smarter and more powerful than whatever guy is reading this.

i can take your girl away from you, sucker -- that kind of thing. something meant to shock.

i really don't want to know the kind of person that would feel moved to put something like that on their car. around here you also see big plastic testicles hanging from the trailer hitch of pickup trucks. this is supposed to be a funny joke.