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.
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)
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