John Calvin Commentary Psalms 34:13

"Keep thy tongue from evil, And thy lips from speaking guile." — Psalms 34:13 (ASV)

Keep thy tongue from evil. The precept David delivers here relates to a very rare virtue: that we should be truthful and free from deceit in our discourse.

Some, indeed, understand it in a much more extended sense, supposing that slander is condemned in this first clause. But it seems to me simpler, and more to the point, to understand this as having the same import as what he repeats in the second clause: that we should not speak deceitfully with our neighbors, so that our words may prove the means of ensnaring them.

And since nothing is more difficult than to regulate our discourse so that our speech may be a true representation of our hearts, David calls upon us to exercise strict and watchful control over it, not allowing it to run riot, lest it cause us to deceive others.