The Hebrew adjective chalaq means smooth, sleek, or slippery — and by extension, flattering or deceptive. It describes both physical smoothness (smooth stones) and the smooth words of a deceiver.
Chalaq is used for the smooth stones David selected for his sling (1 Samuel 17:40) and for the flattering, slippery words of the wicked. Psalm 55:21 vividly contrasts outward smoothness with inner treachery: 'His talk is smooth as butter, yet war is in his heart; his words are more soothing than oil, yet they are drawn swords.' The word warns against the danger of those whose speech is polished but whose hearts are deceptive. True speech (Proverbs 3:3-4) is faithful and true, not slick and smooth.