"Quack is a pejorative term, disparagingly, albeit sometimes defensively, applied by a member of the establishment, the orthodox, regular, professional, credentialed and accepted class to describe the unorthodox, unlicensed, disapproved member of a fringe or irregular group. It is a term of condemnation employed when one wants to belittle another. Above all, the term has become associated with the sellers of medicines and the marketers of medical systems, those with the "true" method of curing specific ills or, in an earlier day, all the ills of mankind.
While the origins of the term are obscure, the term "quack" probably came from the Dutch Quacksalber, a charlatan, mountebank, empiric or itinerant seller of medicine. It may also have been derived from the sounds made by a duck, the term applied to the hawker of nostrums whose excessive zeal in describing the merits of his or her cure may well have sounds similar to the squawking of a duck. The chatter of the quack, in most cases more like torrent s of words, would have been familiar to both town and rural populations even in the ancient periods, for quacks have long been well known in every society. Over the past four hundred years they have been representative figures in folktales, stories and especially in prints, drawings and political caricatures..." –William H. Helfand, from Quack Quack Quack
Detail from "Quid hic nobis lumine satium", c. 1670, Anonymous
Detail from advertisement for Dr. Rock's Tincture, 1738, Anonymous
"The Dance of Death: the Undertaker and the Quack." 1816, by Thomas Rowlandson (from Wellcome Library)
"Nancy Linton: A faithful representation of her actual appearance & condition after having been cured by the use of Swann's Panacea", c. 1833, by C Hullmandel (from a drawing by WH Kearney)
"Singular Effects of the Universal Vegetable Pills on a Green Crocer! A Fact!", 1841, by Charles Jameson Grant
Detail from "The Great Lozenge Maker", 1858, by John Leech - from Punch
"Dr S.B. Collins' Painless Opium Antidote" Advertisement, 1874
"Quackery - Medical Minstrel Performing for the Benefit of Their Former Patients - No other Dead-heads Admitted", 1879, by Joseph Keppler - from Puck
"Death's-Head Doctors - Many Paths to the Grave", 1881, by Joseph Keppler - from Puck
Detail of "Death's-Head Doctors - Many Paths to the Grave", 1881, by Joseph Keppler - from Puck
Detail of "Death's-Head Doctors - Many Paths to the Grave", 1881, by Joseph Keppler - from Puck
"Death in the Pestle", c. 1885, by Henry Nappenbach - from The Wasp
Detail of "Death in the Pestle", c. 1885, by Henry Nappenbach - from The Wasp
"The Travelling Quack", 1889, by Tom Merry
An itinerant medicine vendor known as Medicine Jack carrying his wares in a knapsack on his back. (from Wellcome Library)
"William Radam, Microbes and the Microbe Killer", 1890
"The Great American Fraud, an investigative article by Samuel Hopkins Adams", 1907
Quack advertisement for the cure of cancer, 1912 (from Wellcome Library)
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Unless noted all of these come from Quack, Quack, Quack: The Sellers of Nostrums in Prints, Posters, Ephemera, & Books by William Helfand - @ Open Library [link]
Wellcome Library has a good collection of quackery related images [link]
The excellent blog The Quack Doctor [link]
The Museum of Questionable Medical Devices [link]
see the blog Quack Cogitations [link]
Quack cartoons at cartoonstock [link]
BBC slideshow: Quacks and Cures [link]