The phrase “read them the riot act” is an idiom, meaning to speak angrily toward someone and threaten punishment if the anger-causing behavior does not quickly cease. Like most idioms, the expression’s etymology is not immediately apparent from the words themselves. But unlike many idioms whose history is either unknown, non-literal, or (in the case of “kick the bucket“) probably both, when someone is “read the riot act,” as the saying’s history shows, the threat is — or was — quite real. Literally.
In 1714, likely in response to civil uprisings (“riots”), the Parliament of the United Kingdom passed a statute: “An act for preventing tumults and riotous assemblies, and for the more speedy and effectual punishing the rioters.” It’s shorter, yes still official name was simply “The Riot Act.” A small image of the Act is pictured above; a larger one is available here.
The act allowed local authorities to forcibly disperse any band of “unlawfully” (etc.) assembled group of a dozen or more people — with one catch: fair warning had to be issued to the band of people targeted by the act. Fair warning was given by reading a proclamation, as proscribed by the Act itself. The magic paragraph read as follows:
Our Sovereign Lord the King chargeth and commandeth all persons, being assembled, immediately to disperse themselves, and peaceably to depart to their habitations, or to their lawful business, upon the pains contained in the act made in the first year of King George, for preventing tumults and riotous assemblies. God Save the King!
The Riot Act was rarely employed; however, in 1819, fifteen were killed and hundreds more were injured in the Peterloo Massacre, as military cavalry, with sabers drawn, charged a crowd numbering in the mid-five figures. And while the Riot Act is no longer law in the United Kingdom today (it has been replaced by a statute allowing for the arrest of rioters), the United States has a rarely-used law which is similar to the original British one (more on that here).
Related?: For some reason, when one searches Amazon for “riot act,” this t-shirt comes up.