| tough(adjective, strong, hard, firm, serious) |
| unyielding(adjective, strong, hard, tough, firm) |
| flinty(adjective, strong, hard, tough, firm) |
| inflexible(adjective, strong, hard, tough, firm) |
| adamantine(adjective, strong, hard, tough, firm) |
| hard(adjective, strong, tough, firm, solid) |
| stony(adjective, hard, firm, solid, hostile) |
| obdurate(adjective, strong, hard, tough, cold) |
| rigid(adjective, strong, hard, tough, firm) |
| firm(adjective, strong, hard, tough, solid) |
| unbending(adjective, strong, hard, tough, firm) |
| uncompromising(adjective, set, determined, resolute, unsympathetic) |
| adamant(adjective, strong, hard, tough, resolute) |
| inexorable(adjective, strong, hard, tough, determined) |
| stern(adjective, severe, harsh, serious, resolute) |
| stiff(adjective, set, hard, firm, dead) |
| implacable(adjective, strong, hard, tough, determined) |
| unrelenting(adjective, strong, hard, tough, determined) |
| callous(adjective, cold, cruel, unfeeling, unsympathetic) |
| determined(adjective, strong, hard, tough, resolute) |