The Phrase Finder explains that the this expression means to “Reprimand rowdy characters and warn them to stop behaving badly.” It had its origin in the actual English Riot Act of 1715. Any group of twelve or more unruly citizens had an hour to disperse or be arrested after the Riot Act was read to them. The English government was especially fearful of Jacobite mobs who wanted to overthrow the King and restore Roman Catholic Stuart King James VII of Scotland to the throne.