fetters, shackles
鋜 centers on metal restraints for the feet: fetters and shackles that bind the ankles or legs.
鋜 combines 金 (metal) with 足 (foot), suggesting metal restraints for the feet. The exact historical development is uncertain, but the component combination fits the meaning of fetters or shackles.
Metal 金 around the foot 足: picture heavy iron shackles clamped around both ankles, making every step difficult.
For サク, imagine a prisoner in shackles trying to walk, but the fetters make a sharp 'saku' sound with each step: saku -> サク.