твердый как железо

From LSJ

πολλὰ δ' ἄναντα κάταντα πάραντά τε δόχμιά τ' ἦλθον → and ever upward, downward, sideward, and aslant they went

Source

Russian > Greek

σιδήρεος, σιδάρεος, σιδηροῦς