The idiom “to read the riot act” is used to describe a firm oral reprimand given to someone or a group. For example, one might say “after she read the riot act to the boys for clowning around, they settled down.” Rowdy behavior, rambunctiousness, or rudeness could all be viewed as causes for someone to be reprimanded, sometimes in front of other people to enforce the idea that the warning is indeed serious.
This slang term has its origins in the Riot Act of 1714, a law passed in Britain for the purpose of suppressing civil unrest. During this period in Britain, mobs would regularly gather to protest the government and incite anti-government behaviors. While some mobs started out relatively calmly, with people making speeches and talking among themselves, they sometimes grew out of control. The government also wanted to suppress the appearance of such mobs with the goal of solidifying the position of the king, fearing that the monarchy would be undermined by these gatherings.
In response, Parliament passed the Riot Act. Under the Riot Act, any group of 12 or more people could be deemed a “mob,” and if a magistrate literally read the Riot Act to the mob, they were legally obligated to disperse. Failure to disperse after an hour could be grounds for imprisonment, hard labor, or fines. As one might imagine, the penalties for violating the Riot Act were an incentive for groups to meet in secret so that they would not be caught, and to disperse quickly to avoid penalties.
Well through the 1800s, magistrates used the Riot Act to control unruly groups. By the 1840s, the Riot Act was falling out of favor, although it was not officially repealed until 1973. Along the way, it inspired the slang term, thereby becoming enshrined in the English language, although many modern English speakers are unaware of its history.
The first uses of the phrase in an idiomatic sense date to the early 1800s, right around the time when use of the Riot Act as a tool of discipline declined. Many Britons were familiar with the Riot Act, so it is perhaps not surprising that it came to be used in a joking idiom. Today, when someone is read the riot act, the goal is usually to reprimand him or her for negative behavior, encouraging an apology and a promise not to repeat the behavior.