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.

The 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.

The 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.

The man whose wisdom dwells in his heart, will be found more truly prudent than many who possess shining talents.

wise in heart--who rightly consider duty.
sweetness of the lips--eloquent discourse, persuades and instructs others.

Four 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 = 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" = 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.

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.