Cold iron

Iron has a long and varied tradition in the mythology and folklore of the world.

Cold iron is a poetic term for iron.

Francis Grose's 1811 Dictionary of the Vulgar Tongue defines cold iron as "A sword, or any other weapon for cutting or stabbing." This usage often appears as "cold steel" in modern parlance.

Rudyard Kipling's poem "Cold Iron", found in his 1910 collection of stories Rewards and Fairies, used the term poetically to mean "weapon".

"Cold iron" is historically believed to repel, contain, or harm ghosts, fairies, witches, and other malevolent supernatural creatures. This belief continued into later superstitions in a number of forms:

In his novel Redgauntlet, the Scottish author Sir Walter Scott wrote, "Your wife's a witch, man; you should nail a horse-shoe on your chamber-door."

In modern fantasy, cold iron may refer to a special type of metal, such as meteoric iron or unworked metal. Weapons and implements made from cold iron are often granted special efficacy against creatures such as fairies and spirits.

Horseshoes are considered a good luck charm in many cultures, including those of England, Denmark, Lithuania, and Estonia, and its shape, fabrication, placement and manner of sourcing are all important. A common tradition is that if a horseshoe is hung on a door with the two ends pointing up (as shown here) then good luck will occur. However, if the two ends point downwards then bad luck will occur. Traditions do differ on this point, though. In some cultures, the horseshoe is hung points down (so the luck pours onto you); in others, it is hung points up (so the luck does not fall out); still in others it does not matter so long as the horseshoe has been used (not new), was found (not purchased), and can be touched. In all traditions, luck is contained in the shoe and can pour out through the ends.

...
Wikipedia