A 20th Century term, that referred to a Homosexual who denies being Gay, even to himself. One who denies their own sexual orientation.
Many males deny their attraction to other males, primarily due to social standards, or morality.
In some cultures, admitting to such feelings, can lead to imprisonment or death. (Iran)
Some consider the term to be representative of the extreme barrier put in place by these people, to insure their secret is never revealed.
Hence its reference to Iron.
“Iron Hoof” was 20th Century Cockney rhyme slang as hoof rhymed with poof, short for poofter, a common term for a Homosexual.