| impenetrable(adjective, tough, firm, stiff) |
| iron(adjective, strong, hard, tough) |
| stiff(adjective, firm, harsh, determined) |
| rocky(adjective, firm, stiff, compact) |
| strong(adjective, hard, strength, tough) |
| rigorous(adjective, strong, hard, tough) |
| rugged(adjective, strong, hard, tough) |
| solid(adjective, strong, hard, severe) |
| indurate(adjective, firm, determined, stiff) |
| dense(adjective, strong, hard, tough) |
| strict(adjective, strong, hard, tough) |
| callous(adjective, strong, hard, firm) |
| dyed-in-the-wool(adjective, strong, hard, tough) |
| indomitable(adjective, strong, hard, tough) |
| compact(adjective, strong, hard, firm) |
| compacted(adjective, strong, hard, firm) |
| compressed(adjective, strong, hard, firm) |
| concentrated(adjective, strong, hard, firm) |
| consolidated(adjective, strong, hard, firm) |
| hard-handed(adjective, strong, hard, compact) |