What is Lettres De Cachet?

Legal Definition
Letters issued and signed by the kings of France, and countersigned by a secretary of state, authorizing the imprisonment of a person. It is said that they were devised by P6re Joseph, under the administration of Richelieu. They were at first made use of occasionally as a means of delaying the course of justice ; but during the reign of Louis XIV. they were obtained by any person of sufficient influence with the king or his ministers. Under them, persons were imprisoned for life or for a long period on the most frivolous pretexts, for the gratification of private pique or revenge, and without any reason being assigned for such punishment. They were also granted by the king for the purpose of shielding his favorites or their friends from the consequences of their crimes; and thus were as pernicious in their operation as the protection afforded by the church to criminals in a former age. Abolished during the Revolution of 1789. Wharton.
-- Black's Law Dictionary