מִדַּעְתִּ֕י כִּ֥י קָשֶׁ֖ה אָ֑תָּה וְגִ֤יד בַּרְזֶל֙ עָרְפֶּ֔ךָ וּמִצְחֲךָ֖ נְחוּשָֽׁה׃
Because I knew that thou art obstinate, And thy neck is an iron sinew, and thy brow brass;
Read kabbalistic (mystical) interpretation of Isaiah 48:4: and classical mystical sources.