Albert Barnes Commentary Job 40:4

"Behold, I am of small account; What shall I answer thee? I lay my hand upon my mouth." — Job 40:4 (ASV)

Behold, I am vile: what shall I answer you? – “Instead of being able to argue my cause, and to vindicate myself as I had expected, I now see that I am guilty, and I have nothing to say.” He had argued boldly with his friends. He had, before them, maintained his innocence of the charges which they brought against him, and had supposed that he would be able to maintain the same argument before God.

But when the opportunity was given, he felt that he was a poor, weak man; a guilty and miserable offender. It is a very different thing to maintain our cause before God, from what it is to maintain it before people; and though we may attempt to vindicate our own righteousness when we argue with our fellow human beings, yet when we come to maintain it before God we shall be speechless.

On earth, people vindicate themselves; what will they do when they come to stand before God in the judgment?

I will lay my hand upon my mouth – an expression of silence. Catlin, in his account of the Mandan Indians, says that this is a common custom with them when anything wonderful occurs. Some of them laid their hands on their mouths and remained in this posture by the hour, as an expression of astonishment at the wonders produced by the brush in the art of painting. Compare Job 21:5 and Job 29:9.