Snake oil usually means something is probably fake, will not work, or ineffective: generally speaking it is a scam.
Back in the late 1800's Chinese workers used snake oil as medicine while working here in the States. Traveling doctors would try to sell this stuff. You see those guys portrayed in a lot of old western movies.
Anyway, as it turns out, snake oil is real...but the active ingredient is found only in snakes indiginous to China...so the snakes they were using over here would not work, which led to the belief that snake oil was a scam. These days, it's just become a broad term.
And that is your history lesson for today.