Think of something going down without a single bump. Whole fruit spun until there is nothing left to chew, so the drink slides straight down — that is the picture the name is built on, smooth with a casual -ie on the end. Now move the same picture from a glass to a person: the words come out with no bumps either, nothing catches, nothing is awkward. Which is pleasant in a drink, and makes you wonder a little in a person.
可以想成「一路都不卡」的那種順。整顆水果打到沒有東西可以咬，喝下去一滑到底——名字就是從這個畫面來的：smooth 加上隨口的小尾巴 -ie。接著把同一個畫面從杯子移到人身上：他講話也一樣不卡，沒有停頓，沒有尷尬的地方。在飲料裡，這叫好喝；在人身上，就會讓人想多留一個心眼。
From the word’s history
Wiktionary derives it as smooth + -ie, which is exactly the thread both uses hang on.
資料來源把它記成 smooth 加上 -ie。這條線索正好是兩個意思共用的那一條：滑順。差別只在滑順的是口感還是講話的方式。
Wiktionary via packet evidence (kaikki:3f6f69a190036ae4:676647754): From smooth + -ie.
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