Autonomous weapons are weapons that can fight without pairing with a Meister. While most of weapons can't fight without a meister (like Soul), or inflict minor damage (like Marie), an autonomous weapon can be deadly even without a meister present.
So far, only two autonomous weapons are known: Giriko and Justin. It is probably for this reason, both seem to have developed such a strong rivalry between each other.
Unlike Justin, however, Giriko did have his own self-built meister for a time, in the form of the Oldest Golem. Although the golem contains Arachne's soul, it seems to do little to affect Giriko's combat performance.
We can say, though, that Justin's decision to not have a meister could have been an obliged choice because of his weapon form of a guillotine, which in fact, probably doesn't have as much use in a battle, if he where in his complete weapon form.