In a cavernous manner; with a deep, hollow, echoing quality; spaciously or vastly.
From 'cavernous' (Latin 'cavernosus' from 'caverna') plus '-ly' (adverb suffix); means 'in a cavernous way.'
When someone speaks 'cavernously,' their voice echoes as if emanating from deep inside a cave—it's why opera singers love big, vaulted concert halls.
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