真的嚇人
容器裝滿的狀態，也是這個字最原本的力道。
adjective · C1
So terrible or so ugly that it really does frighten you when you see it or think about it.
駭人的；恐怖的
核心畫面
Think of a container with the word poured into it, because -ful means full of. How full it still is tells you which use you are looking at. Right full: the thing genuinely you. Half full: nothing is , but it is bad enough that the speaker wants a heavy word. Nearly empty but still loud: the word has stopped describing altogether and is only turning the volume up on the noun after it. In a few speakers' English it even drops out of the adjective slot and stands in front of another adjective, where all that is left of the is the loudness.
-ful 是「充滿」，所以 frightful 可以想成一個裝了 fright（驚嚇）的容器。裝得多滿，決定你看到的…更多收起
-ful 是「充滿」，所以 frightful 可以想成一個裝了 fright（驚嚇）的容器。裝得多滿，決定你看到的是哪一個用法。裝滿的時候，那樣東西是真的嚇人。裝一半的時候，事情不嚇人，但糟到說話的人想用重話。快空了卻還很大聲的時候，它已經不在描述了，只是在把後面那個名詞的音量調大。到了某些人的口語裡，它甚至離開形容詞的位置，跑到另一個形容詞前面，這時剩下的只有那份「大聲」。From the word’s history
The source records frightful as fright + -ful, from Middle English frightful meaning afraid and Old English forhtfull meaning fainthearted or timorous. So the older word described someone holding the fright, while today's main uses describe the thing causing it.
來源記載 frightful 是 fright 加上 -ful，來自中古英語的 frightful（害怕的）和古英語的 forhtfull（膽怯的）。也就是說，早期這個字講的是「心裡裝著害怕的人」，現在常見的用法則反過來，講「讓人害怕的東西」。方向翻了一面，容器裡裝的東西沒變。
Wiktionary, frightful, etymology section supplied in packet (kaikki:3f6f69a190036ae4:1024164413)
Two branches
很糟、加強語氣
容器裡的驚嚇越來越少，留下來的是說話者的情緒和音量。
adjective · B2
Very bad or very unpleasant. Used about things such as an accident, the weather, a noise or a journey that upset you or make you uncomfortable.
很糟的；很可怕的