Think of a jar with only one thing in it. Nothing has been stirred in, and nothing has been added to make it go further. Whatever is in the jar is all of it, and it is only that. The word began with food and drink, where adding something cheaper was a way of cheating a buyer. English then took the same jar and used it for feelings and judgements: joy with nothing else stirred into it, nonsense with no truth stirred into it.
From the word’s history
The packet's Wiktionary entry gives the word as un- + adulterated, that is, 'not made impure by adding something'. The idea of something being added in is still what the word is about today, whether the thing added is syrup or doubt.
評估資料裡的 Wiktionary 記的是 un- 加 adulterated，也就是「沒有被摻東西弄壞」。「有沒有東西被加進來」這件事，到今天還是這個字的重點——不管加進來的是糖漿，還是一點懷疑。
Wiktionary, "unadulterated"
登入就可以看每一支怎麼連回核心畫面。