| unflinching(adjective, harsh, cruel, determined, fierce) |
| unforgiving(adjective, harsh, cruel, determined, fierce) |
| unrelenting(adjective, strong, hard, tough, harsh) |
| unsparing(adjective, strong, hard, tough, severe) |
| unyielding(adjective, strong, hard, tough, harsh) |
| vindictive(adjective, harsh, cruel, determined, fierce) |
| adamantine(adjective, strong, hard, tough, sturdy) |
| dense(adjective, strong, hard, tough, sturdy) |
| ferric(adjective, strong, hard, tough, sturdy) |
| ferrous(adjective, strong, hard, tough, sturdy) |
| firm(adjective, strong, hard, tough, severe) |
| hardboiled |
| heavy(adjective, strong, hard, tough, sturdy) |
| immovable(adjective, strong, hard, tough, sturdy) |
| indomitable(adjective, strong, hard, tough, sturdy) |
| insensible(adjective, strong, hard, tough, sturdy) |
| robust(adjective, strong, hard, tough, sturdy) |
| rugged(adjective, strong, hard, tough, sturdy) |
| steel(adjective, strong, hard, tough, sturdy) |
| stern(adjective, serious, authoritarian) |