| unrepenting(adjective, unfeeling, callous, unbending, toughened) |
| unresponsive(adjective, hard, indifferent, unkind, insensitive) |
| unsubmissive(adjective, unfeeling, callous, unbending, toughened) |
| unsusceptible(adjective, hard, indifferent, unkind, unresponsive) |
| unswayable(adjective, tough, firm, fixed, distant) |
| untouched(adjective, calm, cool, steady, tranquil) |
| unyielding(adjective, strong, hard, sound, tough) |
| uptight(adjective, tough, firm, fixed, distant) |
| well-built(adjective, strong, sound, tough, solid) |
| well-founded(adjective, strong, sound, tough, solid) |
| well-made(adjective, strong, sound, tough, solid) |
| accustomed(adjective, unfeeling, unbending, toughened, disciplined) |
| acrimonious(adjective, tough, severe, harsh, cruel) |
| adamant(adjective, strong, hard, tough, fixed) |
| adamantine(adjective, set, strong, hard, tough) |
| anesthetized(adjective, unfeeling, hard-hearted, nowhere, obdurate) |
| antagonistic(adjective, tough, severe, harsh, cruel) |
| austere(adjective, tough, harsh, cruel, grim) |
| backbone(noun, strength, endurance, persistence, toughness) |
| barbarous(adjective, vicious, savage, pitiless, ruthless) |