Joseph Benson's Commentary of the Old and New Testaments - 1 Samuel 17:38
1 Samuel 17:38. Saul armed David with his armour Not that which he was wont to wear himself, for he was so tall it would not have fitted David, but with armour taken out of his armory. The Hebrew word מדיו , madaiv, however, here rendered armour, more properly signifies his vestments, or his garments, and is so translated chap. 1 Samuel 18:4, and in most other places where it occurs. Indeed his armour is distinguished from this, and particularly described in the following words. He... read more
The Pulpit Commentary - 1 Samuel 17:38-40
Naturalness. The facts are— 1 . Saul clothes David with his armour. 2 . David, distrusting its value, puts it aside. 3 . He goes forth to the conflict armed only with a sling and a stone. There is a curious blending of cowardice, prudence, and folly in Saul's conduct. Not daring to fight the foe, he hesitates not to accept a youth; and while providing ordinary armour for his defence, he fails to see that an armed youth would really be at a disadvantage with an armed giant.... read more