Job 41:30 - Matthew Poole's English Annotations on the Holy Bible

According to this translation the sense is, his skin is so hard and impenetrable, that the sharpest stones are as easy to him as the mire, and make no more impression upon him. But the words are and may be otherwise rendered, as continuing the former sense, They (to wit, the arrows, darts, or stones cast at him) are or fall under him, like (which particle is oft understood) sharp shreds, or fragments of stones; he spreadeth sharp pointed things (to wit, the pieces of swords or darts which were flung at him, and broken upon him) upon the mire. The fragments of broken weapons lie as thick at the bottom of the water in the place of the fight as little stones do in the mire, or as they do in a field after some fierce and furious battle. Or thus, With him (or for him, i.e. for his defence) are sharp stones; he spreadeth himself like an arrow or threshing instrument (which is filled and fortified with iron) in the mire or mud in the bottom of the water: so he doth not describe his resting-place, but rather his back, which he not unfitly compares to sharp stones or threshing instruments, because the darts or stones east at him pierce no more into him than they would do into them if they were thrown at them.