| unshakable(adjective, determined, resolute, obstinate, stubborn) |
| hard-nosed(adjective, strong, hard, secure, tough) |
| stony(adjective, person, mean, hard, cool) |
| steadfast(adjective, strong, secure, tough, flat) |
| adamantine(adjective, strong, hard, tough, resist) |
| dogged(adjective, strong, tough, firm, harsh) |
| set(adjective, strong, determined, resolute, obstinate) |
| mulish(adjective, strong, hard, tough, firm) |
| unbendable(adjective, resist, determined, resolute, persistent) |
| tough(adjective, mean, strong, hard, cool) |
| hardened(adjective, person, hard, tough, severe) |
| tenacious(adjective, strong, determined, resolute, obstinate) |
| callous(adjective, person, mean, hard, cool) |
| grim(adjective, mean, strong, hard, tough) |
| implacable(adjective, mean, strong, hard, tough) |
| intractable(adjective, tough, firm, difficult, violent) |
| severe(adjective, mean, strong, hard, tough) |
| unwavering(adjective, strong, secure, flat, certain) |
| perverse(adjective, mean, tough, firm, harsh) |
| stern(adjective, mean, hard, tough, severe) |