To put on armour, Heb. to gird on a girdle, i.e. a military girdle, to which the sword was fastened, 2 Samuel 20:8 1 Kings 2:5. In the border, or, in that border, to wit, of their country, which was towards Edom, which way they understood the kings came. Here they stood probably to defend the passages into their country.
Related Commentaries of 2 Kings 3:21
2 Kings 3:21
21 And when all the Moabites heard that the kings were come up to fight against them, they gatheredc all that were able to put on armour, and upward, and stood in the border.