The quality or state of being honest, sincere, and open in expressing one's true feelings.
Built from 'frankhearted' plus the suffix '-ness,' a common English suffix for creating abstract nouns describing qualities. The construction shows how English can extend a single concept through multiple layers of grammar.
The suffix '-ness' is one of the most productive in English—you can practically add it to any adjective and create a valid noun, which is why English has so many near-synonyms with slightly different flavors of meaning!
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