Charles Ellicott Commentary 1 Samuel 17:5

"And he had a helmet of brass upon his head, and he was clad with a coat of mail; and the weight of the coat was five thousand shekels of brass." — 1 Samuel 17:5 (ASV)

A coat of mail. —More accurately, breastplate of scales.

This armour has sometimes been understood as “chain armour,” but it is more probable that the Philistine armour was made of metal scales, like those of a fish, whose defensive coat was, no doubt, imitated at a very early date by this warlike race, who dwelt on the seashore, and whose life and worship were so closely connected with the great sea. This coat of mail, or corselet, was flexible, and covered the back and sides of the wearer. The weight of the different pieces of the giant’s panoply largely exceeds the weight of medieval suits of armour.