In the mouth of the foolish is a rod of pride. חֹטֶר (choter), "rod," or "shoot," is found also in Isaiah 11:1. From the mouth of the arrogant fool proceeds a growth of vaunting and conceit, accompanied with insolence towards others, for which he is often chastised.
So the tongue is compared to a sword (e.g. Psalms 57:4; Psalms 64:3; Jeremiah 18:18; Revelation 1:16. St. Gregory ('Mor. in Job.,' 24) applies this sentence to haughty preachers, who are anxious to appear superior to other people, and study more to chide and reprove than to encourage; "they know how to smite sharply, but not to sympathize with humility."
Septuagint, "From the mouth of fools cometh a staff of insolence." The lips of the wise shall preserve them—the wise (Proverbs 13:3). These do not abuse speech to insult and injure others; and their words tend to conciliate others, and promote peace and good will (comp.