The Riot Act

This was an actual law enacted in England in the 1700s which allowed the police to disperse crowds that they felt threatened the social order.

The Riot Act had to be “read aloud” to the assembled crowd to warn them of impending arrest. This was the exact text of the act:

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.

So to apply legal force to a crowd, someone had to literally “read the Riot Act.”

At some point, this phrase entered common usage as a metaphor for yelling at someone or dressing them down. I can only assume that the Riot Act was usually read out with some level of anger.