Albert Barnes Commentary Job 40:7

"Gird up thy loins now like a man: I will demand of thee, and declare thou unto me." — Job 40:7 (ASV)

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 yourself 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 you.” 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.