The quality or state of being adequate; the degree to which something is sufficient or satisfactory.
From 'adequate' (from Latin 'adaequatus') plus the Old English suffix '-ness,' which creates abstract nouns from adjectives. This combination became standard in English for converting qualities into nouns.
English speakers love the '-ness' suffix—we add it to almost any adjective to turn it into an abstract concept, so 'adequateness' is just the natural way English makes a noun from the adjective 'adequate.'
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