The quality or state of being fell; cruelty, fierceness, or mercilessness.
From the adjective 'fell' (meaning cruel, fierce, fierce) with the abstract noun suffix '-ness.' This has been in English since at least Middle English, creating an abstract quality from a concrete descriptor.
The '-ness' suffix is one of the most productive in English—you can add it to almost any adjective to get an abstract noun, which is why 'fellness' works perfectly even though it's rarely used today!
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