en_tn/isa/48/04.md

your neck muscles tight as iron, and your forehead like bronze

Yahweh compares the tightness of their neck muscles and the hardness of their foreheads to the hardness of iron and bronze. Here, to have a tight neck or a hard forehead is a metaphor that means the people are stubborn. Alternate translation: "it is as if your necks were iron and your heads were bronze" (See: rc://en/ta/man/jit/figs-simile and rc://en/ta/man/jit/figs-metaphor)