be a mule

1. informal To be extremely stubborn or obstinate. My dad can be a real mule when he makes up his mind about something, even when we can prove that he's wrong. My toddler can be a mule when she's tired. Get ready for tantrums any time she doesn't get what she wants! You'll never get Rich to change his opinion—that guy's a total mule

2. slang To transport contraband, especially illegal drugs, between locations on behalf of a criminal organization. In this poverty-ridden country, some of the most lucrative—and dangerous—work is being a mule. The drug lord has forced several human trafficking victims to be mules for him. I could never be a mule—I have a hard time breaking even the most insignificant of rules!