A rod of pride - i. e., The pride shown in his speech is as a rod with which he strikes down others and himself.

The mouth of the foolish is a rod of pride - The reproofs of such a person are ill-judged and ill-timed, and generally are conveyed in such language as renders them not only ineffectual, but displeasing, and even irritating.

In the mouth of the foolish [is] a (c) rod of pride: but the lips of the wise shall preserve them.
(c) His proud tongue will cause him to be punished.

In the mouth of the foolish is a rod of pride,.... A proud tongue, or a tongue speaking proud and haughty things; with which foolish or wicked men smite others and wound and hurt their reputation and credit, and in the issue hurt themselves also; their tongue is not only a rod to others, but a scourge to themselves, or is the cause of evil coming upon them; such was the tongue of Pharaoh, as Jarchi on the place observes, Exodus 5:2; and of those the psalmist speaks of, Psalm 73:9; and particularly of antichrist, whose mouth is opened in blasphemies against God, and his tabernacle, and his saints, Revelation 13:5;
but the lips of the wise shall preserve them; from speaking such proud and haughty things against God and men; or from being hurt by the tongues of men or their own; yea, what coaxes out of their mouth is confounding and destructive to their enemies, Revelation 11:5.

Pride grows from that root of bitterness which is in the heart. The root must be plucked up, or we cannot conquer this branch. The prudent words of wise men get them out of difficulties.

rod of pride--that is, the punishment of pride, which they evince by their words. The words of the wise procure good to them.

3 In the mouth of the fool is a switch of pride;
But the lips of the wise preserve them.
The noun חטר (Aram. חוּטרא, Arab. khiṭr), which besides here occurs only at Isaiah 11:1, meaning properly a brandishing (from חטר = Arab. khatr, to brandish, to move up and down or hither and thither, whence âlkhṭtâr, the brandisher, poet. the spear), concretely, the young elastic twig, the switch, i.e., the slender flexible shoot. Luther translates, "fools speak tyrannically," which is the briefer rendering of his earlier translation, "in the mouth of the fool is the sceptre of pride;" but although the Targum uses חוטרא of the king's sceptre and also of the prince's staff, yet here for this the usual Hebr. שׁבט were to be expected. In view of Isaiah 11:1, the nearest idea is, that pride which has its roots in the heart of the fool, grows up to his mouth. But yet it is not thus explained why the representation of this proceeding from within stops with חטר cf. Proverbs 11:30). The βακτηρία ὕβρεως (lxx, and similarly the other Greek versions) is either meant as the rod of correction of his own pride (as e.g., Abulwald, and, among the moderns, Bertheau and Zckler) or as chastisement for others (Syr., Targum: the staff of reviling). Hitzig is in favour of the former idea, and thinks himself warranted in translating: a rod for his back; but while גּוה is found for גּאוה, we do not (cf. under Job 41:7 : a pride are the, etc.) find גאוה for גוה, the body, or גּו, the back. But in general it is to be assumed, that if the poet had meant חטר as the means of correction, he would have written גּאותו. Rightly Fleischer: "The tongue is often compared to a staff, a sword, etc., in so far as their effects are ascribed to it; we have here the figure which in Revelation 1:16 passes over into plastic reality." Self-exaltation (R. גא, to strive to be above) to the delusion of greatness is characteristic of the fool, the אויל [godless], not the כּסיל [stupid, dull] - Hitzig altogether confounds these two conceptions. With such self-exaltation, in which the mind, morally if not pathologically diseased, says, like Nineveh and Babylon in the prophets, I am alone, and there is no one with me, there is always united the scourge of pride and of disgrace; and the meaning of 3b may now be that the lips of the wise protect those who are exposed to this injury (Ewald), or that they protect the wise themselves against such assaults (thus most interpreters). But this reference of the eos to others lies much more remote than at Proverbs 12:6; and that the protection of the wise against injury inflicted on them by words is due to their own lips is unsatisfactory, as in this case, instead of Bewahrung [custodia], we would rather expect Vertheidigung [defensio], Dmpfung [damping, extinguishing], Niederduckung [stooping down, accommodating oneself to circumstances]. But also it cannot be meant that the lips of the wise preserve them from the pride of fools, for the thought that the mouth preserves the wise from the sins of the mouth is without meaning and truth (cf. the contrary, Proverbs 13:3). Therefore Arama interprets the verb as jussive: the lips = words of the wise mayest thou keep i.e., take to heart. And the Venet. translates: χείλη δὲ σοφῶν φυλάξεις αὐτά, which perhaps means: the lips of the wise mayest thou consider, and that not as a prayer, which is foreign to the gnome, but as an address to the hearer, which e.g., Proverbs 20:19 shows to be admissible. but although in a certain degree of similar contents, yet 3a and 3b clash. Therefore it appears to us more probable that the subject of 3b is the חכמה contained in חכמים; in Proverbs 6:22 wisdom is also the subject to תשׁמר עליך without its being named. Thus: while hurtful pride grows up to the throat of the fool, that, viz., wisdom, keeps the lips of the wise, so that no word of self-reflection, especially none that can wound a neighbour, escapes from them. The form תּשׁמוּרם is much more peculiar than ישׁפּוּטוּ, Exodus 18:26, and תעבוּרי, Ruth 2:8, for the latter are obscured forms of ישׁפּטוּ and תעברי, while on the contrary the former arises from תּשׁמרם.
(Note: Vid., regarding these forms with ǒ instead of the simple Sheva, Kimchi, Michlol 20ab. He also remarks that these three forms with û are all Milra; this is the case also in a remarkable manner with ישׁפּוּטוּ, vid., Michlol 21b; Livjath Chen ii. 9; and particularly Heidenheim, in his edition of the Pentateuch entitled Mer Enajim, under Exodus 18:26.)
If, according to the usual interpretation, we make שׂפתי the subject, then the construction follows the rule, Gesen. 146, 2. The lxx transfers it into Greek: χείλη δὲ σοφῶν φυλάσσει αὐτούς. The probable conjecture, that תשׁמורם is an error in transcription for תּשׁמרוּם = תּשׁמרנה אתם (this is found also in Luzzatto's Gramm. 776; and Hitzig adduces as other examples of such transpositions of the ו Jeremiah 2:25; Jeremiah 17:23; Job 26:12, and Joshua 2:4, ותצפנו for ותצפון), we do not acknowledge, because it makes the lips the subject with an exclusiveness the justification of which is doubtful to us.

A rod - His insolent speeches as they are like a rod, offensive to others, so, they make a rod for their own backs.