Better, A man of calm (or noble) spirit is a man of understanding.

He that hath knowledge spareth his words,.... Or, "he that knows knowledge" (c); one that is very knowing, has a fund of knowledge in him, "spareth his words"; is generally a man of few words, he thinks much and says little; and though he may be communicative of his knowledge to proper persons, and at proper times, yet never speaks of it in a boasting and ostentatious way: or, he "restrains his words" (d); he puts a bridle on them; and suffers not himself to speak hastily and angrily, and in a reproachful manner, when he is provoked to it;
and a man of understanding is of an excellent spirit; here is a various reading; the "Cetib" is "of a cool spirit" (e); in opposition to a warm fiery spirit; such as was that of the apostles, who were for calling for fire from heaven on those that slighted their master, and, as he told them, knew not what spirits they were of; but a cool spirit is one that is not soon angry, calm, sedate, and not easily provoked to wrath: the "Keri", or marginal reading, is "of an excellent" or "precious spirit" (f); and such an one is a right spirit; a spirit of faith and love, and of a sound mind; and of such a spirit is a man of understanding in things divine and spiritual; to have a spirit of prayer, and to be tenderhearted, and of a sympathizing and forgiving spirit, is to be of an excellent spirit. The Targum is,
"humble in spirit;''
and a meek and quiet spirit is in the sight of God of great price; the Lord has a great regard to such who are of an humble and contrite spirit: with these he dwells, to these he gives more grace; these are like to Christ, and have the fruits of his Spirit, and are very useful and ornamental. The Septuagint, Syriac, and Arabic versions render it, "patient" or "longsuffering"; and to be of a patient spirit is to be of an excellent spirit: such bear afflictions and reproaches quietly; wait God's own time for hearing and helping them, and live in the comfortable expectation of heaven and happiness; and such show themselves to be wise and understanding men.
(c) "qui scit scientiam", Pagninus, Vatablus, Mercerus. (d) "qui cohibet sermones suos", Junius & Tremellius, Piscator. (e) "frigidus spiritu", Junius & Tremellius, Piscator, Cocceius. (f) "pretiosus spiritu", Pagninus, Montanus, Mercerus, Gejerus, Baynus.

A man may show himself to be a wise man, by the good temper of his mind, and by the good government of his tongue. He is careful when he does speak, to speak to the purpose. God knows his heart, and the folly that is bound there; therefore he cannot be deceived in his judgment as men may be.

Prudence of speech is commended as is an excellent or calm spirit, not excited to vain conversation.

27 He that keepeth his words to himself hath knowledge,
And the cool of temper is a man of understanding.
The first line here is a variation of Proverbs 10:19. The phrase ידע דּעת (here and at Daniel 1:4) means to possess knowledge (novisse); more frequently it is בּינה ידע, e.g., Proverbs 4:1, where ידע has the inchoative sense of noscere. In 27b the Kerı̂ is יקר־רוח. Jerome translates it pretiosi spiritus, the Venet. τίμιος τὸ πνεῦμα. Rashi glosses יקר here, as at 1-Samuel 3:1, by מנוע (thus to be read after codd.), retentus spiritu; most interpreters remark that the spirit here comes into view as expressing itself in words. It is scarcely correct to say that יקר דּברים could designate one who is sparing in his words, but יקר־רוּח is, according to the fundamental conception of the verb יקר, gravis spiritu (Schultens), of a dignified, composed spirit; it is a quiet seriousness proceeding from high conscientiousness, and maintaining itself in self-control, which is designated by this word. But the Chethı̂b וקר־רוּח presents almost the same description of character. קר from קרר (of the same root as יקר) means to be firm, unmoveable, καρτερὸν εἶναι, hence to be congealed, frozen, cold (cf. frigus with rigere, rigor), figuratively to be cold-blooded, passionless, quiet, composed (Fl.); cf. post-bibl. קרת רוּח (Arab. ḳurrat‛ain), cooling = refreshing, ἀνάψυξις (Acts 3:20).
(Note: "He has made my eye glowing" (askhn, cf. שׁחין) is in Arab. equivalent to "he has deeply troubled me." The eye of the benevolent is bârid, and in the Semitic manner of expression, with deep psychological significance, it is said that the tears of sorrow are hot, but those of joy cold.)
Whether we read יקר or קר, in any case we are not to translate rarus spiritu, which, apart from the impossibility of the expression, makes 27b almost a tautological repetition of the thought of 27a. The first line recommends bridling of the tongue, in contrast to inconsiderate and untimely talk; the second line recommends coldness, i.e., equanimity of spirit, in contrast to passionate heat.