In a manner resembling bass; with low-pitched quality or in the manner of a bass voice or instrument.
From 'bass' with the adverbial suffix -ly, forming an adverb that describes how something is done with bass-like qualities.
This rare adverb shows how English can create new words on the fly by adding -ly to almost anything, though 'bassly' is uncommon because musicians simply say 'deeply' or 'low' instead.
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