In a witty, humorous, or cleverly amusing manner; with playful humor (archaic usage).
From 'facete' plus '-ly' (Old English adverb-forming suffix). Archaic form paralleling modern 'facetiously'.
English used to have tons of adverbs ending in '-ly' that we've completely abandoned—'facetely' is one of the ghosts of old vocabulary, showing how modern English prefers shorter, simpler words over formal elaborations.
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