From Longman Dictionary of Contemporary Englishbe a law unto himself/herself etcbe a law unto himself/herself etcINDEPENDENT PERSONto behave in an independent way and not worry about the usual rules of behaviour or what other people do or think Boys his age are a law unto themselves. → law
Examples from the Corpus
be a law unto himself/herself etc• She was a law unto herself and did what she wanted to do. 