Keep thy tongue from evil The precept which David here delivers relates to a virtue which is very rare, namely, 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 more simple, and more to the purpose, to understand this as of the same import with what he repeats in the second clause, that we should not speak deceitfully with our neighbors, so as that our words may prove the means of ensnaring them. And since nothing is more difficult than to regulate our discourse in such a manner as that our speech may be a true representation of our hearts, David calls upon us to exercise over it a strict and watchful control, not suffering it to run riot, lest it should prove the occasion of our deceiving others.

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 out life. To love the truth; to speak the truth; to avoid all falsehood, slander, and deceit, will contribute to this, or will be a means which 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 which are not true, or any promises which 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 which does not tend to abridge human life, as there is no virtue which 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 truly, 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.

Keep thy tongue from evil,.... This, and what follows in this verse and Psalm 34:14, point at the things wherein the fear of God shows itself; and suggest, that those who have it, and which is known by these fruits, shall enjoy the desirable and good days before mentioned. The tongue is an instrument of much evil, an unruly member, and needs restraint; and it is from evil, and not from good, it is to be kept; from evil speaking of God, from cursing and swearing; from evil speaking of men, reproaching and reviling them; from filthy speaking, from all obscene and unchaste words, and from all lying ones; for where such evil speaking is indulged, the fear of God cannot be in that man;
and thy lips from speaking guile; hypocritical and deceitful words, speaking with flattering lips and a double heart: some speak bad words in common conversation, through an evil habit and custom; and some speak good words with an ill design; and in neither of them is the fear of God before their eyes, nor in their hearts.

Sins of thought included in those of speech (Luke 6:45), avoiding evil and doing good in our relations to men are based on a right relation to God.

Evil - From all manner of evil speaking, from all injurious, false and deceitful speeches.