In a manner characterized by failure, disappointment, or lack of success; with a tendency toward failing.
From 'failing' (present participle of fail) + -ly (adverbial suffix), creating an adverb meaning in a manner of failing.
English speakers create adverbs by adding '-ly' to adjectives, but this particular combination sounds awkward—it's why we rarely say someone acts 'failingly' and prefer simpler phrases like 'unsuccessfully'!
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