The quality of being extremely fearful, terrible, or causing great dread; the state of inspiring intense fear or anxiety.
From dread (Old English drædan, meaning 'to fear') + -ful (Old English -full, meaning 'full of') + -ness (Old English -nes, forming abstract nouns). The suffix chain builds an abstract noun expressing the quality of being dreadful.
Medieval English speakers layered suffixes to intensify emotions—dreadfulness captures not just fear itself, but the abstract concept of how terrifying something is. It's the difference between being afraid and defining what frightfulness even means.
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