The margin renderings are more literal and give greater emphasis. What is pointed out is not the unfitness of lying lips for the princely-hearted, but the necessity of harmony, in each case, between character and speech.

Excellent speech becometh not a fool - This proverb is suitable to those who affect, in public speaking, fine language, which neither comports with their ordinary conversation, nor with their education. Often fine words are injudiciously brought in, and are as unbecoming and irrelevant as a cart wheel among clockwork.

Excellent speech becometh not a fool,.... A wicked man. Eloquence, or a sublime grand way of speaking, a copiousness and fluency of expression, become not such; because hereby he may be capable of doing more mischief; or such a style is unsuitable to the subject of his discourse, which is nothing but folly and wickedness. The Gospel is excellent speech, sound speech, that cannot be condemned; it treats of excellent things; concerning the person, office, and grace of Christ, and salvation by him; and very unfit is a wicked man to take it into his mouth, talk of it, and declare it;
much less do lying lips a prince; they rather become a fool, as excellent speech does a prince; who neither should speak lies himself, nor encourage, but abhor them in others. The Septuagint, Syriac, and Arabic versions, render it, "a just man": but the word more property signifies a liberal man, as it is rendered in Isaiah 32:8; where it stands opposed to a churl or covetous man: and some Jewish (f) writers think by the "fool" is meant such an one to whom a "lip of abundance" (g), as it may be rendered, is very unsuitable; or to talk of his abundance, when he makes no good use of what he has for himself or others; and so, on the other hand, it is very disagreeable to the character of an ingenuous and liberal man to promise and not perform, and never intended it. It is true of such who are made a "willing" people in the day of Christ's power, Psalm 110:3; where the same word is used as here; of his volunteers; that to speak lies one to another very ill becomes them; or to receive, or to speak, or profess false doctrines; for no lie is of the truth.
(f) Kabvenaki in Mercer. in loc. (g) "labium abundantiae".

A fool, in Solomon's Proverbs, signifies a wicked man, whom excellent speech does not become, because his conversation contradicts it.

Excellent speech--(Compare Margin). Such language as ill suits a fool, as lying (ought to suit) a prince (Proverbs 16:12-13).

The proverbs following, Proverbs 17:7-10, appear to be united acrostically by the succession of the letters ש (שׂ, שׁ) and ת.

7 It does not become a fool to speak loftily,
How much less do lying lips a noble!
As at Isaiah 32:5., נבל and נדיב are placed opposite to one another; the latter is the nobly magnanimous man, the former the man who thinks foolishly and acts profligately, whom it does not become to use lofty words, who thereby makes the impression of his vulgarity so much the more repulsive (cf. Job 2:10). שּׂפת יתר (not יתר, for the word belongs to those which retain their Pathach or Segol, in pausa) is neither elevated (soaring) (Ewald) nor diffuse (Jo. Ernst Jungius in Oetinger: lingua dicax ac sermonem ultra quam decorum verbis extendere solita), rather imperative (Bertheau), better presumptuous (Hitzig) words, properly words of superfluity, i.e., of superabundant self-consciousness and high pretension (cf. the transitive bearing of the Arab. watr with ὑβρίζειν, from ὑπέρ, Aryan upar, Job, p. 363). Rightly Meri, שׂפת נאוה ושׂררה. It produces a disagreeable impression, when a man of vulgar mind and of rude conduct, instead of keeping himself in retirement, makes himself of importance, and weighty in a shameless, impudent manner (cf. Ps. 12:9, where זלּוּת, vilitas, in a moral sense); but yet more repulsive is the contrast, when a man in whom one is justified in expecting nobility of mind, in accordance with his life-position and calling, degrades himself by uttering deceitful words. Regarding the אף כּי, concluding a minori ad majus, we have already spoken at Proverbs 11:31; Proverbs 15:11. R. Ismael, in Bereschith Rabba, at 44:8, reckons ten such conclusions a minori ad majus in the Scriptures, but there are just as many quanto magis. The right accentuation (e.g., in Cod. 1294) is here אף כי־לנדיב, transformed from אף כי־לנדיב, according to Accentuationssystem, xviii. 2.

Excellent speech - Discourse of high and excellent things.