For there is not a word, etc. The words admit a double meaning. Accordingly some understand them to imply that God knows what, we are about to say before the words are formed on our tongue; others, that though we speak not a word, and try by silence to conceal our secret intentions, we cannot elude his notice. Either rendering amounts to the same thing, and it is of no consequence which we adopt. The idea meant to be conveyed is, that while the tongue is the index of thought to man, being the great medium of communication, God, who knows the heart, is independent of words. And use is made of the demonstrative particle lo! to indicate emphatically that the innermost recesses of our spirit stand present to his view. In verse fifth some read -- behind and before thou hast fashioned me; [1] but tsvr, tsur, often signifies to shut up, and David, there can be no doubt, means that he was surrounded on every side, and so kept in sight by God, that he could not escape in any quarter. One who finds the way blocked up turns back; but David found himself hedged in behind as well as before. The other clause of the verse has the same meaning; for those put a very forced interpretation upon it who think that it refers to God's fashioning us, and applying his hand in the sense of an artizan to his work; nor does this suit with the context. And it is much better to understand it as asserting that God by his hand, laid as it were upon men, holds them strictly under his inspection, so that they cannot move a hair's breadth without his knowledge.

Footnotes

1 - Thus the Septuagint have eplasas me, Thou hast formed me. Similar is the rendering of the Syriac. Those who embrace this view take the verb, as if the root were ytsr, yatsar. "But," says Phillips, "it is certain that the root of tsrtny must be tsvr, to afflict, press, besiege. Hence the meaning of the verse is, Thou hast so pressed upon, or besieged me, both behind and before, that I find there is no escaping from thee; Thou hast placed thy hand upon me, so that I am quite in thy power.' The whole passage is a figure, representing God's thorough knowledge of man." -- Phillips. "Thou besettest me behind and before, i.e. thou knowest all my doings as perfectly as if I were begirt by thee on every side." -- Cresswell.

For there is not a word in my tongue - All that I say; all that I have power to say; all that I am disposed at any time to say.
But lo, O Lord, thou knowest it altogether - All that pertains to it. What is "said," and what is "meant." Merely to "hear" what is spoken does not imply necessarily a full knowledge of what is said - for it may be false, insincere, hypocritical. God knows exactly what is said and what is "meant."

There is not a word in my tongue - Although (כי ki) there be not a word in my tongue, behold O Jehovah, thou knowest the whole of it, that is, thou knowest all my words before they are uttered as thou knowest all my thoughts while as yet they are unformed.

For [there is] not a word in my (c) tongue, [but], lo, O LORD, thou knowest it altogether.
(c) You know my meaning before I speak.

For there is not a word in my tongue,.... Expressed by it or upon it, just ready to be spoken; or, as the Targum,
"when there is no word in my tongue:''
so Aben Ezra,
"before it was perfect in my tongue:''
before it is formed there; while it is in the mind, and not expressed, and even before that;
but, lo, O Lord, thou knowest it altogether; the whole of it, from whence it springs; the reason of it, what is designed, or the ends to be answered by it. The Lord knows the good words of his people, which they speak to him in prayer, even before and while they are speaking them; and what they say to one another in private conversation, Isaiah 65:24. See an instance of words known by Christ before spoken, in Luke 19:31.