| Barnes' Notes on the Bible The words point to the conditions of all true growth in wisdom; and he who has the gift of uttering it in winning speech increases it in himself and others.
Gill's Exposition of the Entire BibleThe wise in heart shall be called prudent,.... He that has true wisdom in the inward part; who knows his heart and the haughtiness of it; who has the fear of God in it, which is the beginning of wisdom; who is wise unto salvation, not only knows the scheme of it, but is experimentally acquainted with it; who has not head knowledge and wisdom only, but heart knowledge and wisdom, and behaves wisely in his life and conversation; who is so wise and endued with knowledge, as, out of a good conversation, to show his works with meekness of wisdom; such a man is called, reckoned, accounted, and spoken well of, as a prudent man among all wise and knowing persons; and the sweetness of the lips increaseth learning; who, besides a wise heart and a knowing head, have the gift of elocution; can deliver themselves in a flowing easy style; can clothe their thoughts with proper words, and convey their ideas in clear expressions, in a very edifying and instructive manner: these communicate knowledge to others, and increase it in themselves: for, while they are improving others, they improve themselves and learning also, whether it be divine or human; these are such who are "apt to teach", 1 Timothy 3:2; and if they have proper hearers to attend them, they will "increase in learning", as a just man does, Proverbs 9:9.
Keil and Delitzsch Biblical Commentary on the Old TestamentFour proverbs of wisdom with eloquence: 21 The wise in heart is called prudent, And grace of the lips increaseth learning. Elsewhere (Proverbs 1:5; Proverbs 9:9) הוסיף לקח means more than to gain learning, i.e., erudition in the ethico-practical sense, for sweetness of the lips (dulcedo orationis of Cicero) is, as to learning, without significance, but of so much the greater value for reaching; for grace of expression, and of exposition, particularly if it be not merely rhetorical, but, according to the saying pectus disertos facit, coming out of the heart, is full of mind, it imparts force to the instruction, and makes it acceptable. Whoever is wise of heart, i.e., of mind or spirit (לב equals the N.T. νοῦς or πνεῦμα), is called, and is truly, נבון [learned, intelligent] (Fleischer compares to this the expression frequent in Isaiah, "to be named" equals to be and appear to be, the Arab. du'ay lah); but there is a gift which highly increases the worth of this understanding or intelligence, for it makes it fruitful of good to others, and that is grace of the lips. On the lips (Proverbs 10:13) of the intelligent wisdom is found; but the form also, and the whole manner and way in which he gives expression to this wisdom, is pleasing, proceeding from a deep and tender feeling for the suitable and the beneficial, and thus he produces effects so much the more surely, and beneficently, and richly.
Geneva Study BibleThe wise in heart shall be called prudent: and the {k} sweetness of the lips increaseth learning. (k) The sweet words of consolation which come from a godly heart.
Wesley's Notes 16:21 The wise - He who is truly wise, shall be so accounted by others. Sweetness - Eloquence added to wisdom. Increaseth - Both in himself, for while a man teaches others, he improves himself; and others, who by this means are induced to hear and receive his instructions.
Jamieson-Fausset-Brown Bible Commentary21. wise in heart-who rightly consider duty. sweetness of the lips-eloquent discourse, persuades and instructs others.
Matthew Henry's Concise Commentary16:12. The ruler that uses his power aright, will find that to be his best security. 13. Put those in power who know how to speak to the purpose. 14,15. Those are fools, who, to obtain the favour of an earthly prince, throw themselves out of God's favour. 16. There is joy and satisfaction of spirit, only in getting wisdom. 17. A sincerely religious man keeps at a distance from every appearance of evil. Happy is the man that walks in Christ, and is led by the Spirit of Christ. 18. When men defy God's judgments, and think themselves far from them, it is a sign they are at the door. Let us not fear the pride of others, but fear pride in ourselves. 19. Humility, though it exposes to contempt in the world, is much better than high-spiritedness, which makes God an enemy. He that understands God's word shall find good. 21. The man whose wisdom dwells in his heart, will be found more truly prudent than many who possess shining talents. 22. As waters to a thirsty land, so is a wise man to his friends and neighbours. 23. The wise man's self-knowledge, always suggests something proper to be spoken to others. 24. The word of God cures the diseases that weaken our souls. 25. This is caution to all, to take heed of deceiving themselves as to their souls. 26. We must labour for the meat which endureth to everlasting life, or we must perish.
Matthew Henry's Whole Bible CommentaryVerse 21 Note, 1. Those that have solid wisdom will have the credit of it; it will gain them reputation, and they shall be called prudent grave men, and a deference will be paid to their judgment. Do that which is wise and good and thou shalt have the praise of the same. 2. Those that with their wisdom have a happy elocution, that deliver their sentiments easily and with a good grace, are communicative of their wisdom and have words at will, and good language as well as good sense, increase learning; they diffuse and propagate knowledge to others, and do good work with it, and by that means increase their own stock. They add doctrine, improve sciences, and do service to the commonwealth of learning. To him that has, and uses what he has, more shall be given. | 
