The quality or state of being blackhearted; fundamental wickedness or moral corruption.
Noun form created by adding '-ness' to 'blackhearted,' creating an abstract noun representing the quality. This follows standard English morphology for creating abstract nouns from adjectives.
The '-ness' suffix lets us turn any quality into an abstract noun (kindness, sadness, blackheartedness), which is why English speakers can endlessly create new nouns—we have a productive system of word-building that's been working for over a thousand years.
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