What does Proverbs 26:23 mean?

Clay is a relatively common, coarse substance. A thin coating of glaze makes it appear more attractive but doesn't change the essential nature of the clay. The Hebrew phrasing used here implies painting something with the dross—impure metals—removed when purifying silver. The broad concept at work is when one deceptively coats an inherently cheap or worthless substance. A related English expression is the attempt to "put lipstick on a pig," referring to disguising something disagreeable.

The person with "fervent lips" is one using intense, passionate language or flattery. The meaning is similar to "hypocrisy:" to say one thing while believing something else. Here, it specifically means a person who disguises evil intent using clever words. Careful phrases, flattery, and compelling words can make evil seem attractive, at first. It can also disguise the real intent behind a conversation. Keeping up appearances to appear pious is another way "fervent lips" can disguise sinful hearts. In Matthew 23:25 Jesus condemned hypocritical scribes and Pharisees. He said, "Woe to you, scribes and Pharisees, hypocrites! For you clean the outside of the cup and the plate, but inside they are full of greed and self-indulgence." Their outward appearance misrepresented their foul, inward spiritual decay.

The following verse says this kind of deception indicates "hate." It implies contempt for the person being lied to.