A non-standard or archaic adverbial form meaning 'in a better manner,' though this form is rarely used in modern English.
From 'better' (adjective/adverb) plus the adverbial suffix '-ly.' While grammatically possible, English speakers typically use 'better' itself as an adverb rather than adding '-ly.'
Betterly is a ghost word—it follows all the rules for creating adverbs in English, yet native speakers instinctively reject it because 'better' already functions as an adverb, showing that rules alone don't determine what sounds 'right' in a language.
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