John Calvin Commentary Psalms 120:4

"Sharp arrows of the mighty, With coals of juniper." — Psalms 120:4 (ASV)

The arrows of a strong man sharpened, with coals of juniper. Here the Psalmist further illustrates the malice of those who distress the simple and innocent with their slander, stating that they hurl their injurious reports just as a man might draw an arrow and pierce his neighbor's body with it. He adds that their slander was like coals of juniper, which penetrate more effectively and burn more intensely the substances they touch than coals from any other kind of wood.

The point is that the tongues of these slanderers were inflamed with fiery heat and, as it were, dipped in deadly poison. Such people were all the more inexcusable because, without gaining anything from it, they were driven by an unrestrained passion to inflict deadly harm on others.

Since the Prophet records nothing here that he did not personally experience, it can be inferred that if he, and others like him, had to be assailed by their enemies with lies—which were like arrows to pierce them or coals to burn them—we should not be surprised to see God's most eminent servants tested by similar attacks.