Albert Barnes Commentary Psalms 34:13

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

Keep thy tongue from evil - From speaking wrong things. Always give utterance to truth, and truth alone. The meaning is that this is one of the methods of lengthening life. To love the truth, to speak the truth, and to avoid all falsehood, slander, and deceit will contribute to this, or will be a means that will tend to prolong life, and to make it happy.

And thy lips from speaking guile - Deceit. Do not “deceive” others by your words. Do not make any statements that are not true, or any promises that you cannot and will not keep. Do not flatter others, and do not give utterance to slander. Be a man characterized by the love of truth, and let all your words convey truth, and truth only. It cannot be doubted that this, like all other virtues, would tend to lengthen life, and to make it prosperous and peaceful. There is no vice that does not tend to shorten human life, as there is no virtue that does not tend to lengthen it. But probably the specific idea here is that the way to avoid the hostility of other people, and to secure their favor and friendship, is to deal with them truthfully and thus to live in peace with them.

It is true, also, that God will bless a life of virtue and uprightness, and though there is no absolute certainty that anyone, however virtuous he may be, may not be cut off in early life, yet it is also true that, other things being equal, a man of truth and integrity will be more likely to live long (as he will be more certain to make the most of life) than one who is false and corrupt.