A person who refuses to do something that might or might not be illegal simply because of the perceived implication that it is. This individual will often "call out" another person for performing said illegal behavior. He/she might also report illegal behavior to the popo.
ME: Hey G, want to take David to go gang banging?
FRIEND: Nah Cuz, Dave be a mega fucking Law Dog holmes.
ME: Hey man, want to shoot off some illegal fireworks?
LAW DOG: No way, those are illegal for a reason. They could hurt someone you know?
ME: Alright, then screw off.
LAW DOG: If you shoot those off, I'm going to have to report it. Sorry bro, but laws keep this country together.