4 And your Lord says unto you, “What shall be given you, or what shall be set before you, against the deceitful tongue?”. What shall be given you, that is, as a weapon to oppose to the deceitful tongue, to guard yourself against the deceitful tongue? “Or what shall be set before you?” He asks to try you: for He will answer His own question. For He answers following up his own inquiry, “even sharp arrows of the Mighty One, with coals that desolate, or that lay waste”.
They that desolate, or that lay waste (for it is variously written in different copies), are the same, because by laying waste, as you may observe, they easily lead unto desolation. What are these coals? First, beloved brethren, understand what are arrows. The “sharp arrows of the Mighty One,” are the words of God...What then are the “coals that lay waste?” It is not enough to plead with words against a deceitful tongue and unrighteous lips: it is not enough to plead with words; we must plead with examples also...The word coals, then, is used to express the examples of many sinners converted to the Lord.
Thou hear men wonder, and say, I knew that man, how addicted he was to drinking, what a villain, what a lover of the circus, or of the amphitheatre, what a cheat: now how he serves God, how innocent he has become! Wonder not; he is a live coal. You rejoice that he is alive, whom you were mourning as dead. But when you praise the living, if you know how to praise, apply him to the dead, that he may be inflamed; whosoever is still slow to follow God, apply to him the coal which was extinguished, and have the arrow of God's word, and the coal that lays waste, that you may meet the deceitful tongue and the lying lips.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)