In a way that is dishonest or intended to trick or mislead someone.
From deceit (from Old French deceite, Latin decipere meaning 'to catch, ensnare, beguile') plus the adverb suffix -fully. The root suggests catching someone unaware with falsehood.
The word deceit comes from the image of catching or ensnaring—someone who acts deceitfully is essentially laying a trap for your trust, which is why the word feels so sneaky.
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