| brutal(adjective, mean, hard, tough, severe) |
| callous(adjective, mean, hard, tough, severe) |
| ceaseless(adjective, certain, persistent, constant, reliable) |
| diligent(adjective, persevering) |
| dour(adjective, close, hard, tough, severe) |
| enduring(adjective, certain, determined, steady, resolute) |
| exacting(adjective, tough, severe, harsh, cruel) |
| fell(adjective, animal, mean, harsh, violent) |
| flinty(adjective, close, strong, hard, tough) |
| grinding |
| hard-hearted(adjective, mean, hard, tough, evil) |
| heartless(adjective, mean, severe, evil, nasty) |
| hidebound(adjective, determined, resolute, obstinate, uncompromising) |
| inconvincible(adjective, determined, resolute, uncompromising, unshakeable) |
| laborious |
| opinionated(adjective, determined, resolute, uncompromising, unshakeable) |
| oppressive |
| ossified(adjective, determined, resolute, uncompromising, unshakeable) |
| sedulous |
| self-opinionated(adjective, determined, resolute, uncompromising, unshakeable) |