| rigorous(adjective, strong, hard, tough, severe) |
| harsh(adjective, mean, strong, hard, tough) |
| iron(adjective, mean, strong, hard, tough) |
| pertinacious(adjective, determined, resolute, obstinate, stubborn) |
| pigheaded(adjective, mean, hard, firm, deliberate) |
| stony(adjective, person, hard, firm, solid) |
| unshakeable(adjective, determined, resolute, uncompromising) |
| headstrong(adjective, strong, hard, tough, rough) |
| steely(adjective, strong, hard, tough, firm) |
| unalterable(adjective, set, strong, hard, secure) |
| uncompliant(adjective, resist, determined, resolute, obstinate) |
| unswayable(adjective, determined, resolute, rigid, obstinate) |
| hidebound(adjective, moderate, severe, firm, determined) |
| persistent(adjective, strong, tough, flat, firm) |
| refractory(adjective, hard, tough, rough, firm) |
| remorseless(adjective, harsh, resist, cruel, determined) |
| set in stone(expression, adjective, idiom, mean, hard) |
| solid(adjective, strong, hard, secure, smooth) |
| unswerving(adjective, strong, smooth, tough, firm) |
| austere(adjective, strong, hard, tough, severe) |