Start with dread, the cold feeling of something you do not want to face. Dreadfully once meant to a degree that brings that feeling on. Two halves live in that phrase: the fear, and the sheer size of the degree. English kept the size and let the fear fade, so the word now either points at how badly something was done, or simply makes the next word bigger.
可以從 dread（害怕）開始想：那種你不想面對的冷冷的感覺。dreadfully 原本的畫面是「到了會讓人害怕的程度」。這句話裡其實有兩半：一半是害怕，一半是那個很大的程度。英文把「程度」留了下來，讓「害怕」慢慢淡掉。所以今天這個字要嘛指「做得有多差」，要嘛只是把後面那個字放大。
From the word’s history
Inherited from Middle English dredfully; by its parts it is dreadful plus -ly, and dreadful itself is dread plus -ful.
承自中古英文 dredfully；拆開就是 dreadful 加 -ly，而 dreadful 又是 dread（害怕）加 -ful（充滿）。所以字的零件裡本來就寫著「害怕」與「程度」。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:749685807:8967)
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