| durable(adjective, strong, tough, firm, stiff) |
| forcible(adjective, strength) |
| hard-boiled(adjective, strong, hard, tough, sturdy) |
| hard-core(adjective, strong, hard, tough, sturdy) |
| hardy |
| harsh(adjective, strong, hard, tough, severe) |
| heavy(adjective, strong, hard, tough, sturdy) |
| horny |
| immutable(hard, resolute, rigid, inflexible, strict) |
| resolved(adjective, determined, resolute, uncompromising, unshakeable) |
| rocklike(adjective, firm, stiff, rigid) |
| severe(adjective, strong, hard, tough, harsh) |
| stone |
| strict(adjective, set, strong, hard, tough) |
| unadaptable(hard, resolute, rigid, inflexible, strict) |
| unchangeable(hard, resolute, rigid, inflexible, strict) |
| unmoved |
| unwavering(adjective, determined, resolute, uncompromising, unshakeable) |
| valid(adjective, strength) |
| vigorous(adjective, strength) |