bq. These days, its just a figurative expression meaning to give an individual or a group a severe scolding or caution, or to announce that some unruly behaviour must cease. But originally it was a deadly serious injunction to a rioting crowd to disperse.
Apparently, the Riot Act had to be read out very carefully and without omitting any words, or it would not be effective. During Jacobite riots c. 1715, this Act was passed, making it a felony if a group of twelve or more persons refused to disperse more than an hour after magistrates had told them to do so. The situation perhaps had some resemblance to that in the Ukraine this week.
It’s only in the more general modern meaning that ‘jemandem die Leviten lesen’ is an equivalent. The somewhat angry site GehMirNichAufDenSack.de, in its Floskeln fürs Volk, goes into little detail.
Michael Rowley (on the ITI Gernet mailing list) says it derives from the custom of an 8th century Bishop of Metz, who laid down that his clergy, whose morals he did not consider up to scratch, should have the relevant parts f the book of Leviticus, which treat of the rules of behaviour of the priests and Levites, read to them. The German Wikipedia says that Leviticus 26 was often used for reprimands in the Middle Ages.