I knew that thou art obstinate; literally, hard, or stiff—the adjective used in the phrase translated in our version "stiff-necked." The idea is still more forcibly expressed in the following clause—thy neck is an iron sinew; or rather, a band of iron, as stiff as if it were made-of the hardest metal.
And thy brow brass. The exact simile here used does not occur elsewhere in Scripture. It seems to be the origin of our expressions, "brazen,… brazen-faced," "to brazen a thing out." The forehead may be hardened for a good or for a bad purpose; in obstinacy or in a determination to resist evil (comp.
Isaiah 1:7 and Ezekiel 3:8 with Jeremiah 5:3; Ezekiel 3:7; Zechariah 7:12). Here the hardening is evil, marking defiance and self-will.