Warn or reprimand forcefully or severely, as in When he was caught throwing stones at the windows, the principal read him the riot act. This term alludes to an actual British law, the Riot Act of 1714, which required reading a proclamation so as to disperse a crowd; those who did not obey within an hour were guilty of a felony. [First half of 1800s]

To chastise loudly, or to issue a severe warning: “After the students stormed the administration building, the president of the university came out and read them the riot act.” In England, unruly crowds that did not disperse after the Riot Act was read to them became subject to the force of the law.