| unconquerable(adjective, persistent, obstinate, insistent, dogged) |
| unmovable(adjective, firmness, hardness, solidity, sturdiness) |
| unregenerate(adjective, persistent, obstinate, insistent, dogged) |
| unstoppable |
| wrongheaded(adjective, persistent, obstinate, insistent, dogged) |
| adamantine(adjective, firmness, hardness, solidity, sturdiness) |
| at a standstill(adjective, firmness, hardness, solidity, sturdiness) |
| cast-iron(adjective, firmness, hardness, solidity, sturdiness) |
| dour(adjective, firmness, hardness, solidity, sturdiness) |
| flintlike(adjective, firmness, hardness, solidity, sturdiness) |
| flinty(adjective, firmness, hardness, solidity, sturdiness) |
| frozen(adjective, firmness, hardness, solidity, sturdiness) |
| granitelike(adjective, firmness, hardness, solidity, sturdiness) |
| granitic(adjective, firmness, hardness, solidity, sturdiness) |
| grim(adjective, firmness, hardness, solidity, sturdiness) |
| hard(adjective, firmness, hardness, solidity, sturdiness) |
| hard-core(adjective, firmness, hardness, solidity, sturdiness) |
| immobile(adjective, firmness, hardness, solidity, sturdiness) |
| immotile(adjective, firmness, hardness, solidity, sturdiness) |
| immotive(adjective, firmness, hardness, solidity, sturdiness) |