The quality or state of being diligent; careful, attentive, and persistent effort in work or responsibility.
From diligent plus -ness suffix, which is one of the most productive ways to form abstract nouns in English (from Germanic roots). The -ness suffix directly converts adjectives into nouns.
The -ness suffix is so powerful in English that you can add it to almost any adjective—'fast' becomes 'fastness,' 'kind' becomes 'kindness'—and 'diligentness' shows this rule working perfectly!
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