| intransigent(adjective, determined, resolute, obstinate, uncompromising) |
| iron(adjective, dull, strong, hard, tough) |
| relentless(adjective, strong, hard, tough, cold) |
| resolute(adjective, determined, stiff, resolute, strict) |
| rugged(adjective, strong, hard, tough, sturdy) |
| steadfast(adjective, determined, resolute, strict, obstinate) |
| stubborn(adjective, strong, hard, tough, determined) |
| immovable(adjective, strong, hard, tough, determined) |
| impenetrable(adjective, firm, rigid, unyielding) |
| rigorous(adjective, set, strong, hard, tough) |
| set(adjective, hard, firm, solid, resolute) |
| business-minded |
| frigid(adjective, hostile, unfriendly, aloof, distant) |
| indestructible(adjective, firm, rigid, unyielding) |
| indurate(adjective, hard, firm, solid, stiff) |
| obstinate(adjective, set, hard, stiff, rigid) |
| rock-hard(adjective, firm, rigid, unyielding) |
| rocky(adjective, firm, rigid, unyielding) |
| severe(adjective, strong, hard, tough, harsh) |
| steel(adjective, strong, hard, tough, sturdy) |