In a direct, blunt, or emotionless manner; in a way that shows no variation in tone or inflection.
From 'flat,' meaning level or without expression, plus the adverbial suffix '-ly.' The sense of 'emotionless' emerged in the 1800s to describe speech without feeling.
When someone speaks flatly, their voice becomes almost robotic—the monotone that removes all emotional color is why it's also called 'flat affect' in psychology, making it a word that describes both sound and emotion!
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