The quality or state of being characterized by failure, incompleteness, or inability to succeed.
From 'failing' (present participle of fail) + -ness (noun suffix forming abstract nouns), creating a noun meaning the quality of failing.
Words like 'failingness' show how English lets us turn almost any adjective into an abstract noun—but we rarely use such formed words if we have simpler alternatives, which is why this sounds unusual to our ears!
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