| steadfast(adjective, feature, state, characteristic, strain) |
| refractory(adjective, tough, strain, rough, firm) |
| durable(adjective, strong, strain, firm, stiff) |
| resistant(adjective, characteristic, strong, strain, firm) |
| complex(adjective, difficult, property, tedious, involved) |
| enduring(adjective, strain, firmness, labour, hardiness) |
| impregnable(adjective, strain, firmness, labour, hardiness) |
| rough(adjective, strong, great, tough, severe) |
| unalterable(adjective, strain, firmness, labour, hardiness) |
| burdensome(adjective, dull, severe, rough, difficult) |
| fast(adjective, adverb, strain, firm, intense) |
| invariable(adjective, strain, firmness, labour, hardiness) |
| lasting(adjective, strain, firmness, labour, hardiness) |
| resilient(adjective, strain, firmness, labour, hardiness) |
| settled(adjective, strain, firmness, labour, hardiness) |
| stressful(adjective, difficult, complex) |
| inflexible(adjective, characteristic, mean, strong, tough) |
| abundant(adjective, strain, firmness, labour, hardiness) |
| proof(adjective, strain, firmness, labour, hardiness) |
| stubborn(adjective, characteristic, quality, mean, strong) |