Jamieson, Fausset and Brown on 1 Samuel 17:40

40. brook--wady.

bag--or scrip for containing his daily food.

sling--The sling consisted of a double rope with a thong, probably of leather, to receive the stone. The slinger held a second stone in his left hand. David chose five stones, as a reserve, in case the first should fail. Shepherds in the East carry a sling and stones still, for the purpose of driving away, or killing, the enemies that prowl about the flock.