Cast Iron is a slur used to describe Blacks in a derogatory way. The color of cast iron after weathering or "black" pipe - plumber's term for ungalvanized cast/ductile iron.
The word Cast Iron is a racial slur that has been used historically to degrade and dehumanize blacks. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to blacks. If you are unsure about whether or not it is appropriate to use the word Cast Iron, it is best to err on the side of caution and avoid using it in any situation.