lay down the law

To lay down the law means to scold someone in order to make something very clear to someone in a stern manner. When you are laying down the law to someone you are explaining the rules and laws to them in a blunt and stern manner, sometimes even being angry after they did not understand the rules previously. Laying down the law is a way of making the current situation or rules very clear, primarily to someone who may be disobeying them already.