Real Dictionary

cast iron
absolutely certain and to be believed
noun: Cast iron is an alloy consisting of iron with around 4 percent carbon. It may or may not contain small amounts of other elements.
uncountable noun: Cast iron is iron which contains a small amount of carbon. It is hard and cannot be bent so it has to be made into objects by casting.

cast iron in British English
a hard, unmalleable alloy of iron made by casting: it contains a high proportion of carbon and silicon, has low tensile strength, and is very fluid and fusible when molten
made of cast iron
noun: an alloy of iron, carbon, and other elements, cast as a soft and strong, or as a hard and brittle, iron, depending on the mixture and methods of molding
noun: iron containing so much carbon (1.7 to 4.5 per cent) that it cannot be wrought and must be cast into shape

cast-iron in American English
adjective: made of cast iron