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.
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' noun sense 3 (packet evidence id wikt:snake oil:noun:2)
Two branches
瓶子裡的那個東西
這一支就是畫面本身：一種真的存在、可以擦在關節上的油，以及當年裝在瓶子裡賣的那種成藥。
noun · C1
Oil made from the fat of a snake and sold as a medicine. Travelling sellers in nineteenth-century America promised it could cure almost anything, and it is also used in Chinese medicine for sore joints.
蛇油（藥）
說得很神、其實沒效
把瓶子換成任何東西，畫面就成立了：軟體、投資方案、減肥法。只要它被包裝成解答卻沒有效果，英文就可以叫它 snake oil。
noun · C1
Something that someone claims will solve a problem, when in fact it does not work. It is usually sold or pushed with very big promises.
假藥／唬人的萬靈丹