In a manner involving backbiting; in a way that involves speaking badly about someone who is not present.
From 'backbiting' + the adverbial suffix '-ly'. This adverb form is rare but follows standard English word formation patterns for creating adverbs from gerund nouns.
This adverb is so rarely used that it's almost a test case—modern English prefers 'in a backbiting manner' or 'through backbiting,' showing how some morphologically correct words fade away because they just don't feel natural to speakers.
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