The quality or state of being healsome; the degree to which something promotes healing and health.
From 'healsome' plus '-ness' (noun suffix creating abstract quality). A doubly archaic formation—combining a rare adjective with a standard nominalization suffix to discuss the abstract concept of health-promoting properties.
Creating abstract nouns from obscure adjectives like this shows how English can theoretically build infinite vocabulary—'healsomeness' is grammatically perfect but you'll almost never encounter it outside of extremely old texts!
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