The arrow - Hebrew "the son of the bow." So Lamentations 3:13, margin. This use of the word son is common in the Scriptures and in all Oriental poetry.
Sling-stones - The sling was early used in war and in hunting, and by skill and practice it could be so employed as to be a formidable weapon; see Judges 20:16; 1-Samuel 17:40, 1-Samuel 17:49. As one of the weapons of attack on a foe it is mentioned here, though there is no evidence that the sling was ever actually used in endeavoring to destroy the crocodile. The meaning is, that all the common weapons used by men in attacking an enemy had no effect on him.
Are turned with him into stubble - Produce no more effect on him than it would to throw stubble at him.

The arrow cannot make him flee,.... The skin of the crocodile is so hard, as Peter Martyr says, that it cannot be pierced with arrows, as before observed; therefore it is not afraid of them, nor will flee from them;
slingstones are turned with him into stubble; are no more regarded by him than if stubble was cast at him; not only stones out of a sling, but out of an engine; and such is the hardness of the skin of the crocodile, that, as Isidore says (e), the strokes of the strongest stones are rebounded by it, yea, even it is said to withstand against musket shot (f).
(e) Origin. l. 12. c. 6. (f) Mandelsloe in Harris's Voyages, &c. vol. 1. p. 759.

arrow--literally, "son of the bow"; Oriental imagery (Lamentations 3:13; Margin).
stubble--Arrows produce no more effect than it would to throw stubble at him.

Turned - Hurt him no more than a blow with a little stubble.