In a deafening manner; so as to overwhelm with noise (Scottish/Northern English dialect).
From 'deave' plus '-ly' adverbial suffix. Scottish and Northern English dialectal adverb form, creating an adverb from the regional verb.
Dialectal adverbs like 'deavely' show how complete linguistic systems existed in regional English—full of verbs, adverbs, adjectives all built from the same roots, all now mostly vanished.
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