Nautiterm: (Nautical Terms Explained) : Copper-Bottomed

‘Copper-bottomed’ described ships that were fitted with copper plating on the underside of their hulls. The process was first used on ships of the British Navy in 1761 to defend their wooden planking against attack by Teredo worms a.k.a. Shipworms and to reduce infestations by barnacles. The method was successful in protecting ships’ timbers and in increasing speed and maneuverability and soon became widely used. Before long, ‘copper-bottomed’ began to be used figuratively to refer to anything that was certain and trustworthy.