Fig. one who ignores laws or rules; one who sets one's own standards of behavior. (*Typically: be ~; become ~.) You can't get Bill to follow the rules. He's a law unto himself.Jane is a law unto herself. She's totally unwilling to cooperate.
A person who is totally independent, especially one who ignores established rules. For example, You can't tell Marge how to punctuate; she's a law unto herself. [Second half of 1800s]