A British law, first passed in 1715, which authorised magistrates and other officials to break up "riots and tumults" of twelve or more people. When faced with a riot, a magistrate was legally required to read the act which would then indemnify the magistrate and those under his orders from prosecution for any actions they subsequently took to break up the riot, particularly any maiming or killing of rioters. Legally, after the act had been read, the magistrate was supposed to allow an hour for the crowd to disperse.

a vigorous reprimand; "I read him the riot act"

a former English law requiring mobs to disperse after a magistrate reads the law to them