The quality of being frightful; extreme scariness, horror, or awfulness.
From 'frightful' (adjective) plus the abstract noun suffix '-ness,' creating a noun that represents the quality or degree of being extremely frightening or terrible.
The 'frightfulness' of war is what made the term 'Schrecklichkeit' (frightfulness) famous in World War I—it meant deliberately using terror and violence to break enemy morale.
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