| adamantine(adjective, strong, hard, tough, resist) |
| unshakable(adjective, mean, strong, hard, tough) |
| steadfast(adjective, clear, strong, tough, severe) |
| implacable(adjective, mean, strong, hard, tough) |
| unbendable(adjective, tough, firm, resist, determined) |
| hard(adjective, mean, strong, tough, severe) |
| tough(adjective, mean, clear, strong, hard) |
| unwavering(adjective, strong, flat, firm, certain) |
| grim(adjective, strong, hard, tough, severe) |
| severe(adjective, clear, strong, hard, tough) |
| hard-nosed(adjective, mean, clear, strong, hard) |
| dogged(adjective, mean, hard, tough, firm) |
| inelastic(adjective, firm, determined, stiff, resolute) |
| stern(adjective, hard, tough, severe, rough) |
| intractable(adjective, hard, tough, rough, firm) |
| iron(adjective, mean, strong, hard, tough) |
| remorseless(adjective, tough, harsh, resist, cruel) |
| steely(adjective, strong, hard, tough, firm) |
| uncompliant(adjective, resist, determined, resolute, obstinate) |
| unshakeable(adjective, determined, resolute, uncompromising) |