A target - Rather, "a javelin." as in 1-Samuel 17:45, and placed between the shoulders, as the quiver was.

Greaves of brass upon his legs - This species of armor may be seen on many ancient monuments. It was a plate of brass (though perhaps sometimes formed of laminae or plates, like the mail) which covered the shin or fore part of the leg, from the knee down to the instep, and was buckled with straps behind the leg. From ancient monuments we find that it was commonly worn only on one leg. Vegetius, de Re Militari, says, Pedites Scutati etiam ferreas ocreas in dextris cruribus copebantur accipere. "The foot soldiers, called Scutati, from their particular species of shield, were obliged to use iron greaves on their right legs." One of these may be seen in the monument of the gladiator Buto, in Montfaucon; and another in the Mosaic pavement at Bognor, in Surrey.
A target of brass between his shoulders - When not actually engaged, soldiers threw their shields behind their back, so that they appeared to rest or hang between the shoulders.
There are different opinions concerning this piece of armor, called here כידון kidon. Some think it was a covering for the shoulders; others, that it was a javelin or dart; others, that it was a lance; some, a club; and others, a sword. It is certainly distinguished from the shield, 1-Samuel 17:41, and is translated a spear, Joshua 8:18.

And he had greaves of brass upon his legs,.... Which were a sort of boots, or leg harnesses, which covered the thighs and legs down to the heels; such as Iolaus (k) and the Grecians usually wore, as described by Homer; which are supposed to be double the weight of the helmet, reckoned at fifteen pounds, so that these must weigh thirty pounds of avoirdupois weight:
and a target of brass between his shoulders; the Targum is,"a spear or shield of brass, which came out of the helmet, and a weight of brass upon his shoulders.''Jarchi says the same, and that it was in the form of a spear to defend the neck from the sword; it seems to be a corslet of brass, worn between the helmet and the coat of mail for the defence of the neck, supposed to weigh thirty pounds (l).
(k) Hesiod. Scutum Herc. ver. 122. (l) Vid. Hostii Monomach. David & Goliath, c. 5.

greaves of brass--boots, terminating at the ankle, made in one plate of metal, but round to the shape of the leg, and often lined with felt or sponge. They were useful in guarding the legs, not only against the spikes of the enemy, but in making way among thorns and briers.
a target of brass--a circular frame, carried at the back, suspended by a long belt which crossed the breast from the shoulders to the loins.

And "greaves of brass upon his feet, and a brazen lance (hung) between his shoulders," i.e., upon his back. כּידון signifies a lance, or small spear. The lxx and Vulgate, however, adopt the rendering ἀσπὶς χαλκῆ, clypeus aeneus; and Luther has followed them, and translates it a brazen shield. Thenius therefore proposes to alter כּידון into מגן, because the expression "between his shoulders" does not appear applicable to a spear or javelin, which Goliath must have suspended by a strap, but only to a small shield slung over his back, whilst his armour-bearer carried the larger צנּה in front of him. But the difficulty founded upon the expression "between his shoulders" has been fully met by Bochart (Hieroz. i. 2, c. 8), in the examples which he cites from Homer, Virgil, etc., to prove that the ancients carried their own swords slung over their shoulders (ἀμφὶ δ ̓ ὤμοισιν: Il. ii. 45, etc.). And Josephus understood the expression in this way (Ant. vi. 9, 1). Goliath had no need of any shield to cover his back, as this was sufficiently protected by the coat of mail. Moreover, the allusion to the כּידון in 1-Samuel 17:45 points to an offensive weapon, and not to a shield.

Greaves - Boots.