Ri·ot Act (rīət)
n.
An English law, enacted in 1715, providing that if 12 or more people unlawfully assemble and disturb the public peace, they must disperse upon proclamation or be considered guilty of felony.
Idiom:

read the riot act
To warn or reprimand energetically or forcefully:
The teacher read the riot act to the rowdy class.
Word History: The riot act has been read to far more people than the disturbers of the peace the Riot Act was intended to control. The official Riot Act was enacted by Parliament in 1715 to discourage unlawful assembly and civic turbulence, although the first recorded use of the term Riot Act to refer to this legislation does not appear until 1731. The act provided that if 12 or more people gathered unlawfully or for purposes of disturbing the peace, a portion of the Riot Act would be read to them, and if the assembled did not disperse by one hour after this reading, they would be guilty of felony. The Riot Act, which was not repealed until 1973, became a part of the public consciousness and developed an extended sense in the phrase to read the riot act, meaning “to warn forcefully.” The first use of riot act in this way is found in a work published in 1819: “She has just run out to read the riot act in the Nursery.”

[TahlilGaran] American Dictionary ▲