In a disrespectful, casual, or glib manner, without proper seriousness or consideration.
From the adjective 'flippant' plus the adverb-forming suffix '-ly.' Flippant itself likely derives from the verb 'flip,' suggesting light or careless treatment.
When someone says something 'flippantly,' they're treating serious matters like they're nothing—it's the verbal equivalent of flipping something away dismissively, which perfectly explains why it feels so insulting.
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