Albert Barnes Commentary Job 41:27

"He counteth iron as straw, [And] brass as rotten wood." — Job 41:27 (ASV)

He esteemeth iron as straw - He regards instruments made of iron and brass as if they were straw or rotten wood. That is, they make no impression on him. This will agree better with the crocodile than any other animal. So hard is his skin, that a musket-ball will not penetrate it; see numerous quotations proving the hardness of the skin of the crocodile, in Bochart.