| impregnable(adjective, strain, firmness, labour, hardiness) |
| lasting(adjective, strain, firmness, labour, hardiness) |
| pertinacious(adjective, tough, determined, resolute, obstinate) |
| headstrong(adjective, hard, tough, rough, harsh) |
| enduring(adjective, strong, sound, tough, strain) |
| implacable(adjective, appearance, mean, strong, hard) |
| unbendable(adjective, firm, resist, determined, unreasonable) |
| invariable(adjective, strain, firmness, labour, hardiness) |
| settled(adjective, strain, firmness, labour, hardiness) |
| stout(adjective, strong, sound, tough, strain) |
| fast(adjective, strong, secure, tough, strain) |
| long-standing(adjective, strain, firmness, labour, hardiness) |
| perpetual(adjective, strain, firmness, labour, hardiness) |
| proof(adjective, strain, firmness, labour, hardness) |
| robust(adjective, strong, hard, sound, tough) |
| stationary(adjective, strain, firmness, labour, hardiness) |
| substantial(adjective, strong, hard, sound, tough) |
| sustained(adjective, strain, firmness, labour, hardiness) |
| withstanding(adjective, strain, firmness, labour, hardiness) |
| hard-nosed(adjective, mean, clear, strong, hard) |