Gird up thy loins now like a man - An expression taken from the ancient mode of dress. That was a loose, flowing robe, which was secured by a girdle when traveling, or when one entered upon anything requiring energy; see the notes at Matthew 5:38-41. The meaning here is, "Prepare thyself for the highest effort that can be made. Put forth all your strength, and explain to me what will now be said;" compare the notes at Isaiah 41:21.
I will demand of thee - Hebrew "I will ask of thee." That is, I will submit some questions to you to be answered.
And declare thou unto me - Hebrew "Cause me to know." That is, furnish a satisfactory answer to these inquiries, so as to show that you understand the subject. The object is to appeal to the proofs of divine wisdom, and to show that the whole subject was far above human comprehension.

Gird up thy loins - See Job 38:1-3. Some think that this and the preceding verse have been repeated here from Job 38:1-3, and that several of the words there, here, and Job 42:3, have been repeated, in after times, to connect some false gatherings of the sheets of parchment, on which the end of this poem was originally written. See on Job 40:1 (note), and at the end of the chapter.

Gird up thy loins now like a man: I will demand of thee, and,
declare thou unto me,.... And prepare to give an answer to what should be demanded of him. The same way of speaking is used in Job 38:3; See Gill on Job 38:3.

(See on Job 38:3). Since Job has not only spoken against God, but accused Him of injustice, God challenges him to try, could he govern the world, as God by His power doth, and punish the proud and wicked (Job 40:7-14).

7 Gird up thy loins manfully:
I will question thee, and do thou answer me!
8 Wilt thou altogether annul my right,
Condemn me, that thou mayest be righteous?
9 And hast thou then an arm like God,
And canst thou with the voice thunder like Him?
The question with האף stands to Job 40:2 in the relation of a climax: Job contended not alone with God, which is in itself wrong, let it be whatsoever it may; he went so far as to lose sight of the divine justice in the government of the world, and in order not to be obliged to give up his own righteousness, so far as to doubt the divine. ואם, Job 40:9, is also interrogative, as Job 8:3; Job 21:4; Job 34:17, comp. Job 39:13, not expressive of a wish, as Job 34:16. In the government of the world, God shows His arm, He raises His voice of thunder: canst thou perhaps - asks Jehovah - do the like, thou who seemest to imagine thou couldst govern the world more justly, if thou hadst to govern it? וּבקוּל כּמהוּ are to be combined: of like voice to Him; the translation follows the accents (ובקול with Rebia mugrasch).