A series of letters written by Dean Swift to the people of Ireland, advising them not to take the copper money coined by William Wood, by patent granted by George I. These letters crushed the infamous job, and the patent was cancelled.
Dean Swift signed himself M. B. Drapier in these letters.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Drapier's Letters from Fact Monster: