A snake oil salesman is somebody that sells an item that claims to have some miraculous powers and is usually accompanied by a tremendous amount of hype. Also very often the snake oil salesman has planted accomplices who will claim that the product really works.
There really was such a thing as snake oil; it originated in China. Snake oil was a folk remedy in Chinese medicine that was used primarily to treat joint pain such as arthritis and bursitis.
Throughout the 1800s salesmen traveled the U.S. peddling solutions to just about every medical ill you can think of. As depicted in Mark Twain's Huckleberry Finn, the "doctor" was aided by a shill in the crowd who would, at the appropriate moment, call out that the medicine had cured his ills. Once the unsuspecting public had purchased the con artists' products both the saleman and his partner would quickly slip away before the townspeople discovered the product was worthless. Besides the traveling salesman, newspapers and magazines were another popular method of selling these products.
Of course, snake oil salesmen of the 1800s didn't sell actual snake oil, or if they did, it was rattlesnake oil which is useless. Genuine Chinese snake oil was made from the oil of the Chinese water snake, which is rich in the omega-3 acids which recent research has shown can be effective.
The latter half of the 1800s saw a rise in the popularity of "patent medicines." The term "patent medicine" were drug compounds with colorful names and even more colorful claims.
By the middle of the 1800s the manufacture of patent medicines was a major industry in America. They usually had a lot of alcohol laced with morphine, opium, or cocaine. A lot of the medicines were advertised as good to treat infants and the results could be predictable when giving a child alcohol and narcotics. Remedies were claimed to cure venereal diseases, tuberculosis, colic in infants, indigestion and cancer. Another popular ailment they claimed to cure were "Female complaints."
As mentioned, in the US "snake oil" was often produced using rattesnakes. The Rattlesnake King was Clark Stanley. The former cowboy claimed he had learned about the healing power of rattlesnake oil from Hopi medicine men. Stanley created a huge stir at the 1893 World's Exposition in Chicago when he took a live snake and sliced it open before a crowd of onlookers.
Stanley reached into a sack, took out a snake, slit it open and plunged it into boiling water and when the fat rose to the top, he skimmed it off and claimed it was 'Stanley's Snake Oil.' Rattlesnake oil was nowhere near as effective as Chinese snake oil which had almost triple the amount of a oilas rattlesnake oil.
That was really a moot point though. Stanley's Snake Oil didn't contain any snake oil at all. The Pure Food and Drug Act of 1906 led to Stanley's undoing. A shipment of his oil was seized in 1917 and it was found that it primarily contained mineral oil, beef fat, red pepper and turpentine. As a result Clark Stanley was fined $20 (that's about $429 in today's dollars) for violating the food and drug act and for misbranding his product by falsely and fraudulently representing it as a remedy for all pain.
Vietnamese Snake Wine
Princeton University Snake Oil Article
Snake Oil Salesman Alive and Well in Dr. Oz (Huffington Post article)
Rebirth of Snake Oil Salesmen on Wall Street (Forbes article)
Snake Oil for the 21st Century (Consumer Reports article)