"Excellent speech is not fitting for a fool, much less are lying lips to a prince"
This is the Hebrew word yeter which means rest, abundance, excellency. The Theological Wordbook of the Old Testament says that the root, "... refers to the portion of a quantity which has been divided. Generally it is the smaller part..."
The idea in this verse seems to be that the fool – the selfish, impulsive, and rebellious person – has this ability that they usually don't which is that they know how to talk extremely well and therefore act as though they are wise. The ability to say things in an excellent or persuasive way is an art, and it usually accompanies wisdom. So it is unusual when a person who is selfish, impulsive, and rebellious can use language as though they are full of wisdom.
This is the Hebrew word sapa which means lip, language, speech. Clearly Solomon is saying it is unusual and disconcerting to have a truly selfish person be able to speak as though they cared about other people, to use words to suggest that they are full of knowledge, and to converse about various subjects as though they should be listened to.
Solomon says that it is not fitting. The word fitting is the Hebrew word naweh which means beautiful. This is the idea of conforming to God's standards – what matches with what He put inside of us. Beauty is a result of proportionality and conformity to the accepted norms that God has put within. This is why in certain modern art studios they want to proclaim that the grotesque is beautiful in order to rebel from God's internal standard.
In this case, Solomon is pointing out that occasionally one sees the anomaly of a person whose life does not match their ability with words. This is a fool who has excellent speech. Why is this not proportional or beautiful? Because the two normally would not grow together. Because we evaluate people based upon their speech. What a man says reveals what is in his heart. When the normal clue of how a person talks sends off false signals about what a person is really like, it is not fitting. It sets you back.
Realize what Solomon is saying. If a person is always talking about themselves and what they are good at, then they are proud and you can notice that. If a person always turns the conversation toward things that happened to them and things they remember or what they think about something, then the person is selfish and foolish. By the way, this tendency to redirect every conversation to something that relates to you, or your opinion, or your solution is the most common fault in conversation and listening. Everybody – because of the sin nature – wants to do this. Usually only the wise person has the ability to curb the impulse to turn the conversation to be about themselves. Occasionally an unwise fool has learned to not give way to his selfishness by good active listening skills and the use of information that portrays him/her as full of helpful information. This is rare, but it does happen. Solomon is saying that how a person talks or listens is usually a dead give-away about what kind of person you are dealing with.
much less are lying lips to a prince
In the same way that good listening skills and insightful information is not expected from a truly selfish person, so it is not expected or desired to have a leader be a manipulative deceiver.
This is the Hebrew word seqer which is the word for lie. It is clear that Solomon is really trying to make the point in this second part of the proverb. A young up-and-coming leader will be tempted to lie to increase their power and prestige in order to shorten the time before they have more power and more riches. But this ends with the person cutting out their chances to be a great leader. People have a hard time following a person when they cannot trust what comes out of their mouth.
It is not beautiful; it is not right to have a young leader be a manipulative, deceptive, untruthful person. If a young leader uses words to deceive, to gain power, or uses words to shirk responsibility for their own actions, they will not be a prince or an up-and-coming leader for long. They will often be shoved aside in favor of someone who does not lie or deceive. And even if they do become a leader, their leadership will be diminished because less people will be able to clearly follow.
Remember, it is not fitting; it is not right; it is not beautiful for a person who wants to be a leader to be a liar. Solomon is saying that this is a rare occurrence. So don't believe that you will get there that way. Learn to live with the truth. Be comfortable with the responsibility that comes from speaking and dealing with the way things really are and not some fairy tale you can spin to make yourself look good that will come back to bite you.