| Clue | Source | Date |
|---|---|---|
| Like an uncooperative mule | Universal | 20 Mar 2023 |
| Uncooperative | Wall Street Journal | 19 Mar 2022 |
| Obstinate | The Washington Post | 04 May 2021 |
| Obstinate | LA Times Daily | 04 May 2021 |
| Stubborn | LA Times Daily | 15 Apr 2021 |
| Stubborn | The Washington Post | 15 Apr 2021 |
| Uncooperative | Wall Street Journal | 28 Nov 2020 |
| Stubborn, like a mule | USA Today | 08 Mar 2019 |
| Stubborn, like a mule | ||
| Uncooperative, like a mule | USA Today | 03 Dec 2018 |
| Uncooperative, like a mule | ||
| Like a stubborn mule | ||
| Contrary | ||
| Obstinate | ||
| Pigheaded | ||
| Ornery | ||
| Stubborn | ||
| Obstinate | ||
| Obstinate | ||
| Stubborn | ||
| Obstinate | ||
| Obstinate, as a mule | ||
| Like a cantankerous critter | ||
| Mulish | ||
| Contrary | ||
| Stubborn | ||
| Recalcitrant | ||
| Contrary. | ||
| Mulish. |