| Solution | Length | |
|---|---|---|
| 4 letters | ||
| grim | 4 | |
| sure | 4 | |
| hard | 4 | |
| firm | 4 | |
| 5 letters | ||
| stiff | 5 | |
| tight | 5 | |
| rigid | 5 | |
| stern | 5 | |
| tough | 5 | |
| 6 letters | ||
| steely | 6 | |
| strong | 6 | |
| severe | 6 | |
| sturdy | 6 | |
| strict | 6 | |
| 7 letters | ||
| puritan | 7 | |
| austere | 7 | |
| extreme | 7 | |
| unmoved | 7 | |
| adamant | 7 | |
| 8 letters | ||
| stalwart | 8 | |
| puristic | 8 | |
| ungiving | 8 | |
| rigorous | 8 | |
| stubborn | 8 | |
| hardline | 8 | |
| hardcore | 8 | |
| 9 letters | ||
| allornone | 9 | |
| hardnosed | 9 | |
| insistent | 9 | |
| unbending | 9 | |
| ROCKBOUND | 9 | |
| tenacious | 9 | |
| 10 letters | ||
| ROCKRIBBED | 10 | |
| UNYIELDING | 10 | |
| IRONWILLED | 10 | |
| UNAFFECTED | 10 | |
| FLATFOOTED | 10 | |
| BRASSBOUND | 10 | |
| relentless | 10 | |
| 11 letters | ||
| HARDHITTING | 11 | |
| unalterable | 11 | |
| straitlaced | 11 | |
| unrelenting | 11 | |
| puritanical | 11 | |
| uncompliant | 11 | |
| PROCRUSTEAN | 11 | |
| 12 letters | ||
| UNCHANGEABLE | 12 | |
| 13 letters | ||
| STRAIGHTLACED | 13 | |
| single-minded | 13 | |