| immune(adjective, strain, firmness, labour, hardiness) |
| lingering(adjective, strain, firmness, labour, hardiness) |
| permanent(adjective, strain, firmness, labour, hardiness) |
| rock-solid(adjective, hard, strain, firmness, compact) |
| inelastic(adjective, hard, tough, firm, solid) |
| mulish(adjective, mean, strong, hard, tough) |
| severe(adjective, appearance, mean, clear, strong) |
| stern(adjective, appearance, mean, hard, easy) |
| grim(adjective, appearance, mean, strong, hard) |
| steely(adjective, strong, hard, tough, firm) |
| iron(adjective, mean, strong, hard, tough) |
| perverse(adjective, mean, hard, tough, firm) |
| staunch(adjective, clear, strong, sound, tough) |
| unflinching(adjective, mean, strong, severe, flat) |
| unshakeable(adjective, determined, resolute, uncompromising) |
| bullheaded(adjective, strong, hard, tough, firm) |
| flinty(adjective, strong, hard, tough, severe) |
| harsh(adjective, mean, strong, hard, tough) |
| pigheaded(adjective, mean, hard, tough, firm) |
| remorseless(adjective, mean, harsh, resist, cruel) |