Smooth and slippery, or clever and smooth in a way that seems somewhat dishonest or inauthentic.
From Old English 'slician' meaning to smooth or make slippery. The word evolved from describing physical smoothness to describing people who are smooth-talking or too polished to be trustworthy.
A 'slick' salesman is smooth and persuasive like a wet surface is smooth—and this metaphorical connection stuck because something too smooth feels unnatural and makes you suspicious, just like you'd suspect a 'too perfect' pitch is hiding something.
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