snake oil n.

"phony cure, quack remedy," 1927, American English, from the use of oil derived from the fat of snakes (especially the rattlesnake) as a folk remedy in the rural regions of the U.S. Snake oil in this sense is attested by 1858: it was said to heal rheumatism and gout in Georgia, but was touted as a cure for deafness in rural Pennsylvania.

Professional pharmacy journals began to condemn it early 20c., not because it was quackery but because products sold under the name had no real snake oil in them.

What is known as snake oil is usually a combination which is handed out by the dealer to satisfy the demand of some credulous customer. A genuine oil of course is that which is obtained by "trying out" the fat of a snake, usually the rattlesnake, and to preserve their faces druggists sometimes employ a small proportion of such oil in preparing the weird mixtures dispensed by them. [The Practical Druggist, July 1912]