| sturdy(adjective, strong) |
| tenacious(adjective, strong, tough, firm, solid) |
| brassbound(adjective, resist, determined, resolute, persistent) |
| cast-iron |
| chain(noun, verb, free, restraint, bond) |
| chains(noun, shackles, manacles) |
| die-hard(adjective, resist, determined, resolute, persistent) |
| handcuff(noun, verb, free, bond, shackles) |
| harsh(adjective, characteristic, mean, hard, tough) |
| immovable(adjective, mean, strong, hard, tough) |
| metal |
| remorseless(adjective, resist, determined, resolute, persistent) |
| resolute(adjective, hard, determined, resolution, rigid) |
| steadfast(adjective, strong, tough, firm, determined) |
| tin(noun, element) |
| unbendable(adjective, resist, determined, resolute, persistent) |
| wrought iron(expression, idiom, thing, property, hardness) |
| Fe |
| able-bodied(adjective, strong) |
| bone(strength, hardness) |