Definition of cold iron:
(n) :
(mythology) a (possibly fictional) substance harmful to fairies; depending on the story, ordinary iron or iron treated in some way
(n) :
(slang, fishing, Scotland, Ireland) A bad omen.
(n) :
Alternative form of cold steel (“swords, knives, bladed weapons”). [(military, archaic or historical) A sword, bayonet or similar weapon made of steel.]