You may have heard someone describing a chewing-out as "...then he read me the Riot Act." Or maybe your parents threatened to read the Riot Act to you. We kind of know what it means when they say that, but what's behind the idiom? At one time, it had to have made perfect sense to everyone, or else it wouldn't have fallen into common usage.
The idiom, which has been in use for centuries, is generally thought to mean the admonishment of a person or persons who have committed an error in judgment. But the origin of the term "riot act" concerns a very particular wrongdoing—an unlawful public assembly that peace officers of the 16th century fought with a pre-written warning to disperse or face serious repercussions. Like death.
So, it was a legally-worded warning. Strange that an act from England in 1715 would still be used as a common metaphor in the USA today, considering the First Amendment and all. Read how the Riot Act came about and what happened to it at Mental Floss.