The term 'stubborn as a mule' is often used to describe someone who is very determined and unwilling to change their opinion. This phrase can be broken down into two crossword solutions: 'OBDURATE' (8 letters) and 'ORNERY' (6 letters). OBDURATE means 'stubbornly refusing to change one's opinion or course of action, ' while ORNERY means 'having a bad temper; irritable.' Both words are used to describe someone who is stubborn and unwilling to change their opinion.