Picture a man standing on a wagon in an old American town, holding up a small bottle. He says the oil inside comes from snakes, and that it will fix your back, your cough and your bad luck. People pay; the bottle does nothing; by the time anyone notices, the wagon is already in the next town. Every use of snake oil in English still carries that bottle — a very large promise on the outside, and nothing inside.
From the word’s history
The literal product is documented as a nineteenth-century American patent medicine that claimed to contain snake fat, offered as a remedy for many different ailments. The modern dismissive use keeps the shape of that transaction rather than the substance of the oil.
字面上的商品，是十九世紀美國一種宣稱含蛇脂的瓶裝成藥，號稱能治各種毛病。今天罵人的用法，留下來的不是那瓶油，而是那筆交易的形狀：大承諾、沒效果、賣的人先走一步。
Wiktionary, "snake oil"
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