Obdurate is a formal word meaning stubborn. If you want to major in English, but your parents are obdurate that you should go premed, they might go so far as to threaten not to pay your tuition.
This adjective descends from Latin obdurare "to harden." A near synonym is adamant, from Latin adamas "hard metal, diamond." So both of these synonyms derive from the quality of hardness being associated with a stubborn personality.