A snake oil salesman is someone who sells something phony — typically a medicine that promises to cure all that ills you.
Funny enough, though, the original snake oil actually worked! In the 1860s, when Chinese laborers were building the Transcontinental Railroad in the United States, they brought with them a ointment made from Chinese Water snake that they’d rub on their sore, tired, muscles at the end of the day for some relief. They shared this cure with their fellow workers.
Unfortunately, the men who made their livelihood traveling from place to place selling proprietary and patented medicine didn’t appreciate the “natural” competition, so they ridiculed the ointment by calling it and every other cure-all snake oil. Eventually the name caught on and the peddler of such wares became a stock character in Western movies and literature: a traveling “doctor” with dubious credentials, selling some medicine (such as snake oil) with boisterous marketing hype, often supported by pseudo-scientific evidence.