In a hidden, secret way that is not open or easily noticed by others.
From 'covert,' which comes from Old French 'coverte,' the feminine past participle of 'covrir' meaning 'to cover.' The suffix '-ly' converts it to an adverb describing how an action is performed.
Espionage and psychology research shows that humans are terrible at detecting covert behavior—we miss about 50% of intentional deception even when we're explicitly looking for it, which is why magicians and spies succeed by hiding things in plain sight.
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