In a humble, modest, or submissive manner; with a lowered or diminished demeanor.
From 'demiss' (adjective) + '-ly' (adverbial suffix). A standard English adverbial formation from the Latin-derived adjective.
Adverbs like 'demissly' are disappearing from modern English because we prefer simpler words like 'humbly,' but you'll still find it in historical texts where formal politeness mattered more.
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