| indefatigable |
| inelastic(adjective, determined, resolute, obstinate, uncompromising) |
| iron-willed(adjective, determined, resolute, uncompromising, unshakeable) |
| perennial(continuity, diuturnity) |
| perseverant |
| regular(adjective, persistent, constant, continuous, endless) |
| resistant(adjective, determined, resolute, obstinate, uncompromising) |
| robust(adjective, strong, hard, tough, sturdy) |
| rude(adjective, hard, severe, sharp, nasty) |
| single-minded(adjective, tough, firm, harsh, cruel) |
| strong-willed(adjective, determined, resolute, uncompromising, unshakeable) |
| tireless |
| unabated(adjective, merciless) |
| unchangeable(adjective, hard, resolute, rigid, inflexible) |
| unrelieved |
| unstoppable(adjective, harsh, cruel, determined, fierce) |
| untiring |
| vicious(adjective, mean, evil, nasty, harsh) |
| astringent(adjective, strong, hard, tough, severe) |
| authoritarian(adjective, strong, hard, tough, severe) |