Prudent Predators: Someone who promotes individual rights and pretends to adhere to them, but secretly violates them whenever he expects to get away with it.
When someone learns about an ethics of rational self-interest, they'll often ask why they shouldn't steal, kill, or otherwise violate rights when they're sure they can get away with it. If someone leaves their wallet in a room, why not take some money? If you encounter someone you dislike in a dark alley with no witnesses, why not kill them? They suggest that someone who fully acts in their self-interest will be a prudent predator, doing whatever gives them some marginal value despite any damage it does to others. The idea is that if they're not caught, they benefit. And if they benefit, it must be in their self-interest.
There are many different arguments against the notion of prudent predators. For instance, if everyone accepted it, there would be a breakdown in the harmony of interests. Or even if you think you might not get caught, you don't really know. Or that it violates the trader principle. Or that it violates the virtue of pride. Or how spending all of your time looking for ways of sneaking in a crime distracts you from more productive ventures. And there are many more.