Snake oil is historically a beverage or ointment concocted from various inert ingredients (commonly
alcohol) and sold as a cure for various (if not all) diseases and ailments. Often sold with boisterous marketing promoted by travelling "doctors" with dubious credentials, and perhaps an accomplice in the crowd who will attest to the value of the product. Sometimes seen in films depicting the
United States of the 19th and early 20th centuries: often the snake-oil salesperson has to leave town in a hurry when the inhabitants start to realize that they have been conned.
In current usage snake oil is used as a metaphor for a product in which the speaker has no faith, or at best cannot see how it might work even if it does.