User blog comment:Phantomlink959/V3C4 preview/@comment-24891101-20151126033136/@comment-226878-20151127092525

"Balls of steel ring any bells?" I like that, it's humorous. Though seriously, using it to compare to "metal-headed" is rather nonsensical. "Balls of steel" is an alteration of "brass balls" which itself derives inevitably of the phrase "balls off a brass monkey" in reference to how whether affected the construct. This is attested as being used early as the 1840. Being thick of head refers to the thicker bones of the less intellectually-inclined Neanderthalensis.

Meanwhile, I can't find any indication at anyone has ever mindlessly referred to someone as having a metal head. Although an argument could be made due to Ironwood's military standing the phrase is meant to insult his nature as a soldier: helmets were at one time made of metal.