The quality of being sudden or steep; the state of happening without warning or preparation.
From 'abrupt' plus the English suffix '-ness', which creates abstract nouns describing qualities. This pattern dates back to Old English and is one of our most productive word-building systems.
Adding '-ness' to adjectives to make qualities is so automatic in English that we barely notice it—but it's been our default strategy for 1,000+ years, long before we took on Latin terminology.
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