| heavy(adjective, strong, hard, tough, sturdy) |
| immovable(adjective, strong, hard, tough, sturdy) |
| implacable(adjective, strong, hard, tough, sturdy) |
| indomitable(adjective, strong, hard, tough, sturdy) |
| inexorable(adjective, strong, hard, tough, sturdy) |
| inflexible(adjective, strong, hard, tough, sturdy) |
| insensible(adjective, strong, hard, tough, sturdy) |
| leaded |
| metallic |
| mortified(adjective, strong, hard, tough, sturdy) |
| mulish(adjective, strong, hard, tough, sturdy) |
| obdurate(adjective, strong, hard, tough, sturdy) |
| platinum |
| relentless(adjective, strong, hard, tough, sturdy) |
| rigid(adjective, strong, hard, tough, sturdy) |
| rigorous(adjective, strong, hard, tough, sturdy) |
| robust(adjective, strong, hard, tough, sturdy) |
| rock hard(expression, adjective, strong, hard, tough) |
| rough(adjective, strong, hard, tough, sturdy) |
| rugged(adjective, strong, hard, tough, sturdy) |