Albert Barnes Commentary Deuteronomy 33:25

"Thy bars shall be iron and brass; And as thy days, so shall thy strength be." — Deuteronomy 33:25 (ASV)

The strength and firmness of Asher is as if he were shod with iron and brass . The territory of this tribe probably contained iron and copper. Compare the marginal reference.

As your days, so shall your strength be - that is, “your strength” (some prefer “your rest”) “shall be continued to you as long as you shall live: you shall never know feebleness and decay.”