stubborn as a mule

1. Extremely obstinate, as in He's stubborn as a mule about wearing a suit and tie. This simile evokes the proverbial stubbornness of mules, whose use as draft animals was once so common that the reputation for obstinacy can hardly be as warranted as the term indicates. [Early 1800s]

Related Words

• hard-boiled 
• hardheaded 
• no-nonsense 
• tough-minded 
• uncompromising