A smooth-talking or flattering person; someone who uses honeyed words to persuade.
From butter + mouth, using butter as a metaphor for smooth, slippery speech (as in 'butter up'). This metaphorical use reflects how butter is slippery and smooth.
The phrase 'butter up' originates from the idea that flattery greases the wheels of persuasion—just like butter makes things slip and slide, sweet talk helps ideas slide more easily into acceptance.
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