What you think it means: criminal
What it really means: someone immune to prosecution
In historical legal systems, an outlaw is a person declared as outside the protection of the law. In pre-modern societies, this takes the burden of active persecution of a criminal from the authorities. Instead, the criminal is withdrawn all legal protection, so that anyone is legally empowered to persecute or kill them. Outlawry was thus one of the harshest penalties in the legal system, since the outlaw had only himself to protect himself, but it also required no enforcement on the part of the justice system. In early Germanic law, the death penalty is conspicuously absent, and outlawing is the most extreme punishment, presumably amounting to a death sentence in practice.In other words, an outlaw is someone who has committed a crime so heinous that even the people who hunt down criminals for a living no longer want anything to do with him. They declare him an outlaw, and so not only do they no longer need to prosecute him for further crimes, but they don't have to defend him from other criminals. By reserving this treatment for only the most dangerous of criminals, it simultaneously almost guarantees that the outlaw will be killed (either by law-abiding vigilantes or by other criminals), and also reinforces the idea within the culture that a lawless society is dangerous (because everyone who is an outlaw is a particularly dangerous criminal).