The quality or state of being disastrous; the degree to which something causes great damage or harm.
From disastrous + -ness (noun suffix forming abstract nouns). This noun form follows standard English word-building patterns that emerged in Middle English.
Abstract nouns like 'disastrousness' are interesting because they let you discuss the quality of something separate from specific events—you can talk about the disastrousness of climate change without pointing to one particular disaster.
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