You can hold one picture for the whole of frothy: a thick layer of tiny bubbles sitting on top of a liquid. It fills a lot of space, it is mostly air, and it does not last. Every other use keeps some part of that picture. Take away the weight and you get a story that is fun and gone by morning. Take away the value and you get a price that has been pumped up with air.
From the word’s history
Wiktionary gives the formation plainly: frothy is froth plus the ending -y, which turns a noun into an adjective meaning full of that thing. So the literal reading, full of froth, is the oldest layer, and the other two uses are that same picture moved onto books and onto prices.
維基詞典把構詞講得很清楚：frothy 就是 froth（泡沫）加上 -y（有…的、充滿…的）。所以字面上的「滿是泡沫」是最底層的意思，另外兩個用法都是同一張圖被搬到作品和價格上。這裡是構詞，不是猜測，可以放心當成事實講。
Wiktionary 'frothy' (kaikki extract): etymology tree English froth + English -y (via Middle English -y, Old English -iġ).
登入就可以看每一支怎麼連回核心畫面。