Green’s Dictionary of Slang

ironheaded adj.

stupid.

1917		Boys’ Life Nov. 30: You rusty, iron-headed boob!
1922		Chariton Courier (Keytesville, MO) 3 Nov. 3/4: Think we’re a bunch of boarding-house girls, you ironheaded idiot?
1941		LeMay, Bennett & Lasky Reap Wild Wind [film script] No one but an ironheaded sap [HDAS].