In a manner that is applicable; in a way that is relevant or suitable for application to a particular situation.
From 'applicable' (Latin 'applicabilis') plus '-ly' (Old English adverbial suffix). Standard English formation converting the adjective to describe how something is done.
The adverb 'applicably' is rarely used because the adjective 'applicable' often appears where the adverb would go—English economizes by using fewer adverbs than the grammar technically allows!
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