Set a watch, O Lord, before my mouth; keep the door of my lips (comp. Psalms 39:1). David's was a hasty, impetuous temper, which required sharp control. He strove to "keep his own mouth with a bridle"—to " be dumb with silence, and hold his peace"—but this was not always possible for him of his own unassisted strength.
He therefore makes his prayer to God for the Divine help.