Gird up now thy loins like a man - To gird up the loins, is a phrase which has allusion to the mode of dress in ancient times. The loose flowing robe which was commonly worn, was fastened with a girdle when men ran, or labored, or engaged in conflict; see the notes at Matthew 5:38-41. The idea here is, “Make thyself as strong and vigorous as possible; be prepared to put forth the highest effort.” God was about to put him to a task which would require all his ability - that of explaining the facts which were constantly occurring in the universe. The whole passage is ironical. Job had undertaken to tell what he knew of the divine administration, and God now calls upon him to show his claims to the office of such an expositor. So wise a man as he was, who could pronounce on the hidden counsels of the Most High with so much confidence, could assuredly explain those things which pertained to the visible creation. The phrase “like a man” means boldly, courageously; compare the notes at 1 Corinthians 16:13.
I will demand of thee, and answer thou me - Margin, as in Hebrew, “make me known.” The meaning is, “I will submit some questions or subjects of inquiry to you for solution. Since you have spoken with so much confidence of my government, I will propose some inquiries as a test of your knowledge.”