Cold iron is a poetic and archaic term for iron, referring to the fact that it feels cold to the touch. In modern usage the term has been most associated with folkloric beliefs that iron, like silver, could ward off ghosts, fairies, witches, and/or other allegedly malevolent supernatural creatures.
"Cold iron" is sometimes asserted to repel, contain, or harm ghosts, fairies, witches, and/or other malevolent supernatural creatures. This belief continued into later superstitions in a number of forms:
Faeries and iron:
Iron, particularly "cold iron", was employed as a protective substance or charm against faeries. In various folklores, supernatural creatures are held to hold an aversion to iron or even be harmed by the touch of iron.
Make more sense now.
I mean heck, one of the few deterrents against the Nix(essentially the top fairy in swedish folklore) is Cold Iron.