1. a vigorous reprimand; - Example: "I read him the riot act"
2. a former English law requiring mobs to disperse after a magistrate reads the law to them;
WordNet (r) 3.0 (2006):
riot act
n 1: a vigorous reprimand; "I read him the riot act"
2: a former English law requiring mobs to disperse after a
magistrate reads the law to them