A dodgy "doctor"
The word "quack" is frequently applied to inexperienced doctors or peddlers of snake oil, miraculous medicines, or at worst, medical conmen.
The etymology is interesting, a shortening of the Dutch expression "quacksalver", meaning one who prattles on about his salves. The word was used during the sixteenth century to describe the travelling doctors and purveyors of miracles, but was shortened to "quack" in later years.
Some have claimed that it is linked to the use of quicksilver (mercury) to treat some diseases (for example syphilis), but while this is a tempting one (due to alchemical connections), it is doubtful.
The typical quack doctor would arrive in a town or village, enthusiastically singing the praises of their elixir, often using a "plant" in the audience who would take the medicine and pronounce himself cured. They would sell their "cure-all" and then duck out of town, moving on to dupe the rubes elsewhere.
They were successful because they played on the fears and medical ignorance prevalent in an age when few people, even doctors, understood the mechanisms of the human body and illness. The arrival of modern research brought better-informed and qualified doctors, and the travelling quack doctors gradually slipped into oblivion.
They have not disappeared altogether, however - the rise the New Age thinking and pseudoscience has brought a new generation of quack into the limelight again - John Sladek uncovered many in his book The New Apocrypha, a wonderful read.
Gritchka reminds me that duckspeak is used in Orwell's Newspeak