| indestructible(adjective, firm, rigid, unyielding, steely) |
| merciless(adjective, person, mean, hard, rough) |
| solid(adjective, rigid) |
| unalterable(adjective, strong, hard, secure, severe) |
| unappeasable(adjective, mean, harsh, cruel, determined) |
| unbreakable(adjective, firm, rigid, unyielding, steely) |
| unfaltering(adjective, strong, firm, resolute, persistent) |
| unflinching(adjective, strong, flat, firm, harsh) |
| unresponsive(adjective, person, cold, indifferent, feelings) |
| unswerving(adjective, strong, tough, firm, determined) |
| unsympathetic(adjective, mean, hard, tough, harsh) |
| willful(adjective, tough, determined, resolute, obstinate) |
| stanch(verb, resolute) |
| austere(adjective, strong, hard, tough, severe) |
| concrete(adjective, rigid, hardness) |
| hanging tough(expression, adjective, mean, hard, determined) |
| hard-boiled(adjective, person, mean, strong, hard) |
| hard-hearted(adjective, mean, hard, tough, harsh) |
| hidebound(adjective, determined, resolute, persistent, obstinate) |
| ossified(adjective, determined, resolute, uncompromising, unshakeable) |