Thou hast beset me behind and before - The word rendered “beset” - צור tsûr - means properly to press; to press upon; to compress. It has reference commonly to the siege of a city, or to the pressing on of troops in war; and then it comes to mean to besiege, hem in, closely surround, so that there is no way of escape. This is the idea here - that God was on every side of him; that he could not escape in any direction. He was like a garrison besieged in a city so that there was no means of escape. There is a transition here (not an unnatural one), from the idea of the Omniscience of God to that of His Omnipresence, and the remarks which follow have a main reference to the latter.
And laid thine hand upon me - That is, If I try to escape in any direction I find thine band laid upon me there. Escape is impossible.