knife, sword
釖 centers on a metal blade: a knife or sword. The two meanings are direct applications of the same idea of a cutting weapon.
釖 combines 金 (metal) and 刀 (blade), straightforwardly forming a metal blade. The exact historical development is uncertain, but the component meaning is clear.
Metal 金 on the left and a blade 刀 on the right: a metal blade is a knife or sword.
For トウ, imagine a tough metal blade: tough -> トウ, and the sword is strong and sharp.