| indefatigable(adjective, resolute, untiring) |
| mulish(adjective, mean, strong, hard, tough) |
| single-minded(adjective, tough, firm, intense, cruel) |
| standing pat(adjective, determined, resolute, obstinate, uncompromising) |
| steely(adjective, strong, hard, tough, cold) |
| stringent(adjective, set, hard, severe, sharp) |
| uncompliant(adjective, tough, firm, resist, determined) |
| unrelieved(adjective, continuous) |
| unshakeable(adjective, determined, resolute, uncompromising) |
| unstoppable(adjective, cruel, continuous, merciless) |
| decided(adjective, resolution, resolute) |
| endless(adjective, continue, determined, steady, persistent) |
| hard-hearted(adjective, mean, hard, tough, evil) |
| hardheaded(adjective, strong, hard, tough, cold) |
| incompliant(adjective, resist, determined, resolute, persistent) |
| intractable(adjective, appearance, strong, tough, firm) |
| tireless(adjective, resolute) |
| unbroken(adjective, severe, violent, pure, vicious) |
| unfeeling(adjective, animal, mean, hard, severe) |
| unflagging(adjective, continuous) |