Albert Barnes Commentary Psalms 139:3

"Thou searchest out my path and my lying down, And art acquainted with all my ways." — Psalms 139:3 (ASV)

You compass my path ... - Margin, “winnowest.” The Hebrew word—זרה zârâh—properly means “to scatter,” to cast loosely about—as the wind does dust; and then, to winnow—namely, by throwing grain, when it is thrashed, up to the wind (Isaiah 30:24; Jeremiah 4:11; Ruth 3:2).

Then it means “to winnow out;” that is, to winnow out all the chaff and leave all the grain—to save all that is valuable.

So here it means that God, as it were, “sifted” him (Amos 9:9; Luke 22:31).

When it is said that he did this in his path and his lying down, it is meant that he did it in every way; altogether; entirely.

And are acquainted with all my ways - All the paths that I tread; the whole course of my life. All that I do, in all places and at all times, is fully known to you.