“O Jehovah, set a guard to my mouth; keep watch on the door of my lips. Do not let my heart turn aside to any evil thing, to practice deeds in wickedness with men who practice iniquity; and do not let me eat of their delicacies” (Psalm 141:3-4).
John Calvin (not a blanket-agreement with Calvin; see: http://www.outsidethecamp.org/norefcal.htm) writes:
“As David was liable to be hurt at the unbridled and unprincipled rage of his enemies, so as to be tempted to act in a manner that might not be justifiable, he prays for divine direction, and not that he might be kept back from manual violence merely, but that his tongue might be restrained from venting reproach, or words of complaint. Even persons of the most self-possessed temper, if unwarrantably injured, will sometimes proceed to make retaliation, through their resenting the unbecoming conduct of their enemies. David prays accordingly that his tongue might be restrained by the Lord from uttering any word which was out of joint…Next he seeks that his heart be kept back from every mischievous device that might issue in revenge. The words added — that I may not eat of their delicacies, are to be understood figuratively, as a petition that he might not be tempted by the prosperity which they enjoyed in sin to imitate their conduct. The three things mentioned in the context are to be connected; and it may be advisable to consider each of them more particularly. Nothing being more difficult than for the victims of unjust persecution to bridle their speech, and submit silently and without complaint to injuries, David needed to pray that his mouth might be closed and guarded — that the door of his mouth might be kept shut by God, as one who keeps the gate watches the ingress and egress — נצרה, nitsrah, being the imperative of the verb, rather than a noun.
He next subjoins that God would not incline his heart to an evil thing; for דבר, dabar, is here, as in many other places, used to signify a thing. Immediately after he explains himself to mean, that he would not desire to strive with them in wickedness, and thus make himself like his enemies. Had that monk of whom Eusebius makes mention duly reflected upon this resolution of David, he would not have fallen into the silly fallacy of imagining that he had shown himself the perfect scholar by observing silence for a whole term of seven years. Hearing that the regulation of the tongue was a rare virtue, he betook himself to a distant solitude, from which he did not return to his master for seven years; and being asked the cause of his long absence, replied that he had been meditating upon what he had learned from this verse. It would have been proper to have asked him at the same time, whether during the interim he had thought none, as well as spoken none. For the two things stand connected the being silent, and the being free from the charge of evil thoughts. It is very possible that although he observed silence, he had many ungodly thoughts, and these are worse than vain words. We have simply alluded in passing to this foolish notion, as what may convince the reader of the possibility of persons running away with a word torn from its connection, and overlooking the scope of the writer.
In committing himself to the guidance of God, both as to thoughts and words, David acknowledges the need of the influence of the Spirit for the regulation of his tongue and of his mind, particularly when tempted to be exasperated by the insolence of opposition. If, on the one hand, the tongue be liable to slip and too fast of utterance, unless continually watched and guarded by God; on the other, there are disorderly affections of an inward kind which require to be restrained. What a busy workshop is the heart of man, and what a host of devices is there manufactured every moment! If God do not watch over our heart and tongue, there will confessedly be no bounds to words and thoughts of a sinful kind, — so rare a gift of the Spirit is moderation in language, while Satan is ever making suggestions which will be readily and easily complied with, unless God prevent.”